diff --git a/include/gametypes.h b/include/gametypes.h
new file mode 100644
index 0000000000..242fc9d9db
--- /dev/null
+++ b/include/gametypes.h
@@ -0,0 +1,67 @@
+#ifndef GUARD_GAMETYPES_H
+#define GUARD_GAMETYPES_H
+
+#include "gba/types.h"
+
+//
+// This header includes typedefs for fields that commonly appear throughout 
+// the codebase, and which ROM hacks might benefit from being able to widen.
+//
+// These typedefs include the underlying type in their name for two reasons:
+//
+//  - Game Freak wasn't fully consistent about field widths throughout 
+//    their codebase. For example, when Region Map Sections are persistently 
+//    stored in savedata, they're stored as 8-bit values; but much of the 
+//    codebase handles them as 16-bit values.
+//
+//  - Although Pokemon Emerald doesn't come close to maxing out RAM, it *does* 
+//    use nearly all of its EEPROM. That is: the vanilla game uses 96% of the 
+//    flash memory available for storing players' save files, leaving 2172 
+//    bytes to spare within each of the game's two save files (primary and 
+//    backup). These spare bytes are not contiguous: SaveBlock1 can only grow 
+//    by 84 bytes, and SaveBlock2 can only grow by 120 bytes, with the rest 
+//    of the free space located after the player's PC-boxed Pokemon.
+//
+//    With so little flash memory to spare, keeping track of how much space 
+//    you're using is vital -- and so is arranging struct members to minimize 
+//    compiler-inserted padding. It's easier to deal with this when you can 
+//    see these types' widths at a glance.
+//
+// Accordingly, this file generally doesn't contain just single types, but 
+// rather families of types. For example, Region Map Sections are saved as 
+// u8s within the player's save file, but are sometimes handled as u16s or 
+// even s16s and ints; and so there are multiple typedefs for Map Sections 
+// corresponding to each of these underlying types, and each typedef has a 
+// name which indicates the underlying type.
+//
+// For a given family of typedefs, the smallest one should be considered 
+// the "real" or "canonical" type. Continuing with Map Sections as our 
+// example, the smallest type is an 8-bit integer, and so any values that 
+// can't fit in an 8-bit integer will be truncated and lost at some point 
+// within the codebase. Therefore mapsec_u8_t is the "canonical" type for 
+// Map Sections, and the larger typedefs just exist to describe situations 
+// where the game handles Map Sections inconsistently with that "canon."
+//
+
+// Map Sections are named areas that can appear in the region map. Each 
+// individual map can be assigned to a Map Section as appropriate. The 
+// possible values are in constants/region_map_sections.h.
+//
+// If you choose to widen Map Sections, be aware that Met Locations (below) 
+// are based on Map Sections and will also be widened.
+typedef u8  mapsec_u8_t;
+typedef u16 mapsec_u16_t;
+typedef s16 mapsec_s16_t;
+typedef s32 mapsec_s32_t;
+
+// Met Locations for caught Pokemon use the same values as Map Sections, 
+// except that 0xFD, 0xFE, and 0xFF have special meanings.
+//
+// Because this value appears inside every Pokemon's data, widening it will 
+// consume a lot more space within flash memory. The space usage will be 
+// greater than you expect due to how Pokemon substructs are laid out; you 
+// would have to rearrange the substructs' contents in order to minimize 
+// how much more space a wider Met Location would consume.
+typedef mapsec_u8_t metloc_u8_t;
+
+#endif //GUARD_GAMETYPES_H
diff --git a/include/global.fieldmap.h b/include/global.fieldmap.h
index 1207bd80ea..90f2080d71 100644
--- a/include/global.fieldmap.h
+++ b/include/global.fieldmap.h
@@ -167,7 +167,7 @@ struct MapHeader
     /* 0x0C */ const struct MapConnections *connections;
     /* 0x10 */ u16 music;
     /* 0x12 */ u16 mapLayoutId;
-    /* 0x14 */ u8 regionMapSectionId;
+    /* 0x14 */ mapsec_u8_t regionMapSectionId;
     /* 0x15 */ u8 cave;
     /* 0x16 */ u8 weather;
     /* 0x17 */ u8 mapType;
diff --git a/include/global.h b/include/global.h
index f0882710e5..bdfc426fb9 100644
--- a/include/global.h
+++ b/include/global.h
@@ -5,6 +5,7 @@
 #include <limits.h>
 #include "config.h" // we need to define config before gba headers as print stuff needs the functions nulled before defines.
 #include "gba/gba.h"
+#include "gametypes.h"
 #include "constants/global.h"
 #include "constants/flags.h"
 #include "constants/vars.h"
diff --git a/include/global.tv.h b/include/global.tv.h
index 9c3902e7cc..a2721b06b4 100644
--- a/include/global.tv.h
+++ b/include/global.tv.h
@@ -226,7 +226,7 @@ typedef union // size = 0x24
         /*0x04*/ u8 filler_04[2];
         /*0x06*/ u16 itemIds[SMARTSHOPPER_NUM_ITEMS];
         /*0x0C*/ u16 itemAmounts[SMARTSHOPPER_NUM_ITEMS];
-        /*0x12*/ u8 shopLocation;
+        /*0x12*/ mapsec_u8_t shopLocation;
         /*0x13*/ u8 playerName[PLAYER_NAME_LENGTH + 1];
         /*0x1B*/ //u8 padding;
     } smartshopperShow;
@@ -241,7 +241,7 @@ typedef union // size = 0x24
         /*0x0E*/ u16 species2;
         /*0x10*/ u8 nBallsUsed;
         /*0x11*/ u8 outcome;
-        /*0x12*/ u8 location;
+        /*0x12*/ mapsec_u8_t location;
         /*0x13*/ u8 playerName[PLAYER_NAME_LENGTH + 1];
         /*0x1B*/ //u8 padding;
     } pokemonTodayFailed;
@@ -267,7 +267,7 @@ typedef union // size = 0x24
         /*0x04*/ u16 caughtPoke;
         /*0x06*/ u16 steps;
         /*0x08*/ u16 species;
-        /*0x0A*/ u8 location;
+        /*0x0A*/ mapsec_u8_t location;
         /*0x0B*/ u8 language;
         /*0x0C*/ u8 filler_0C[7];
         /*0x13*/ u8 playerName[PLAYER_NAME_LENGTH + 1];
@@ -282,7 +282,7 @@ typedef union // size = 0x24
         /*0x04*/ u8 badgeCount;
         /*0x05*/ u8 nSilverSymbols;
         /*0x06*/ u8 nGoldSymbols;
-        /*0x07*/ u8 location;
+        /*0x07*/ mapsec_u8_t location;
         /*0x08*/ u16 battlePoints;
         /*0x0A*/ u16 mapLayoutId;
         /*0x0C*/ u8 language;
@@ -309,7 +309,7 @@ typedef union // size = 0x24
         /*0x00*/ u8 kind;
         /*0x01*/ bool8 active;
         /*0x02*/ u16 item;
-        /*0x04*/ u8 location;
+        /*0x04*/ mapsec_u8_t location;
         /*0x05*/ u8 language;
         /*0x06*/ u16 mapLayoutId;
         /*0x08*/ u8 filler_08[11];
@@ -336,7 +336,7 @@ typedef union // size = 0x24
         /*0x00*/ u8 kind;
         /*0x01*/ bool8 active;
         /*0x02*/ u16 lastOpponentSpecies;
-        /*0x04*/ u8 location;
+        /*0x04*/ mapsec_u8_t location;
         /*0x05*/ u8 outcome;
         /*0x06*/ u16 caughtMonBall;
         /*0x08*/ u16 balls;
@@ -505,7 +505,7 @@ struct GabbyAndTyData
     /*2BA6*/ u16 mon2;
     /*2BA8*/ u16 lastMove;
     /*2BAA*/ u16 quote[1];
-    /*2BAC*/ u8 mapnum;
+    /*2BAC*/ mapsec_u8_t mapnum;
     /*2BAD*/ u8 battleNum;
     /*2BAE*/ u8 battleTookMoreThanOneTurn:1;
              u8 playerLostAMon:1;
diff --git a/include/landmark.h b/include/landmark.h
index 395905033a..f5feb407bc 100644
--- a/include/landmark.h
+++ b/include/landmark.h
@@ -1,6 +1,6 @@
 #ifndef GUARD_LANDMARK_H
 #define GUARD_LANDMARK_H
 
-const u8 *GetLandmarkName(u8 mapSection, u8 id, u8 count);
+const u8 *GetLandmarkName(mapsec_u8_t mapSection, u8 id, u8 count);
 
 #endif // GUARD_LANDMARK_H
diff --git a/include/overworld.h b/include/overworld.h
index bde6896569..8ac0ddc81e 100644
--- a/include/overworld.h
+++ b/include/overworld.h
@@ -121,8 +121,8 @@ u8 GetLastUsedWarpMapType(void);
 bool8 IsMapTypeOutdoors(u8 mapType);
 bool8 Overworld_MapTypeAllowsTeleportAndFly(u8 mapType);
 bool8 IsMapTypeIndoors(u8 mapType);
-u8 GetSavedWarpRegionMapSectionId(void);
-u8 GetCurrentRegionMapSectionId(void);
+mapsec_u8_t GetSavedWarpRegionMapSectionId(void);
+mapsec_u8_t GetCurrentRegionMapSectionId(void);
 u8 GetCurrentMapBattleScene(void);
 void CleanupOverworldWindowsAndTilemaps(void);
 bool32 IsOverworldLinkActive(void);
diff --git a/include/pokemon.h b/include/pokemon.h
index 53f7f02fd0..3d75157f91 100644
--- a/include/pokemon.h
+++ b/include/pokemon.h
@@ -131,7 +131,7 @@ struct PokemonSubstruct2
 struct PokemonSubstruct3
 {
  /* 0x00 */ u8 pokerus;
- /* 0x01 */ u8 metLocation;
+ /* 0x01 */ metloc_u8_t metLocation;
 
  /* 0x02 */ u16 metLevel:7;
  /* 0x02 */ u16 metGame:4;
diff --git a/include/pokenav.h b/include/pokenav.h
index 553d2ad0f6..b548824234 100644
--- a/include/pokenav.h
+++ b/include/pokenav.h
@@ -17,7 +17,7 @@ struct PokenavMonListItem
 struct PokenavMatchCallEntry
 {
     bool8 isSpecialTrainer;
-    u8 mapSec;
+    mapsec_u8_t mapSec;
     u16 headerId;
 };
 
@@ -410,7 +410,7 @@ void FreeMatchCallSubstruct1(void);
 int IsMatchCallListInitFinished(void);
 int GetNumberRegistered(void);
 struct PokenavMatchCallEntry *GetMatchCallList(void);
-u16 GetMatchCallMapSec(int index);
+mapsec_u16_t GetMatchCallMapSec(int index);
 bool32 ShouldDrawRematchPokeballIcon(int index);
 void ClearRematchPokeballIcon(u16 windowId, u32 tileOffset);
 int GetMatchCallTrainerPic(int index);
@@ -419,7 +419,7 @@ const u8 *GetMatchCallMessageText(int index, bool8 *newRematchRequest);
 u16 GetMatchCallOptionCursorPos(void);
 u16 GetMatchCallOptionId(int optionId);
 void BufferMatchCallNameAndDesc(struct PokenavMatchCallEntry *matchCallEntry, u8 *str);
-u8 GetMatchTableMapSectionId(int rematchIndex);
+mapsec_u8_t GetMatchTableMapSectionId(int rematchIndex);
 int GetIndexDeltaOfNextCheckPageDown(int index);
 int GetIndexDeltaOfNextCheckPageUp(int index);
 bool32 IsRematchEntryRegistered(int rematchIndex);
diff --git a/include/region_map.h b/include/region_map.h
index 2bca9e7f7d..abdb44b16c 100644
--- a/include/region_map.h
+++ b/include/region_map.h
@@ -26,7 +26,7 @@ enum {
 };
 
 struct RegionMap {
-    /*0x000*/ u16 mapSecId;
+    /*0x000*/ mapsec_u16_t mapSecId;
     /*0x002*/ u8 mapSecType;
     /*0x003*/ u8 posWithinMapSec;
     /*0x004*/ u8 mapSecName[20];
@@ -99,14 +99,14 @@ void InitRegionMap(struct RegionMap *regionMap, bool8 zoomed);
 u8 DoRegionMapInputCallback(void);
 bool8 UpdateRegionMapZoom(void);
 void FreeRegionMapIconResources(void);
-u16 GetRegionMapSecIdAt(u16 x, u16 y);
+mapsec_u16_t GetRegionMapSecIdAt(u16 x, u16 y);
 void CreateRegionMapPlayerIcon(u16 tileTag, u16 paletteTag);
 void CreateRegionMapCursor(u16 tileTag, u16 paletteTag);
-bool32 IsEventIslandMapSecId(u8 mapSecId);
-u8 *GetMapName(u8 *dest, u16 regionMapId, u16 padLength);
-u8 *GetMapNameGeneric(u8 *dest, u16 mapSecId);
-u8 *GetMapNameHandleAquaHideout(u8 *dest, u16 mapSecId);
-u16 CorrectSpecialMapSecId(u16 mapSecId);
+bool32 IsEventIslandMapSecId(mapsec_u8_t mapSecId);
+u8 *GetMapName(u8 *dest, mapsec_u16_t regionMapId, u16 padLength);
+u8 *GetMapNameGeneric(u8 *dest, mapsec_u16_t mapSecId);
+u8 *GetMapNameHandleAquaHideout(u8 *dest, mapsec_u16_t mapSecId);
+mapsec_u16_t CorrectSpecialMapSecId(mapsec_u16_t mapSecId);
 void ShowRegionMapForPokedexAreaScreen(struct RegionMap *regionMap);
 void PokedexAreaScreen_UpdateRegionMapVariablesAndVideoRegs(s16 x, s16 y);
 void CB2_OpenFlyMap(void);
diff --git a/src/data/region_map/region_map_layout.h b/src/data/region_map/region_map_layout.h
index 95a4e94f31..32ce91724d 100644
--- a/src/data/region_map/region_map_layout.h
+++ b/src/data/region_map/region_map_layout.h
@@ -1,4 +1,4 @@
-static const u8 sRegionMap_MapSectionLayout[MAP_HEIGHT][MAP_WIDTH] = {
+static const mapsec_u8_t sRegionMap_MapSectionLayout[MAP_HEIGHT][MAP_WIDTH] = {
     {MAPSEC_NONE, MAPSEC_ROUTE_114, MAPSEC_ROUTE_114, MAPSEC_FALLARBOR_TOWN, MAPSEC_ROUTE_113, MAPSEC_ROUTE_113, MAPSEC_ROUTE_113, MAPSEC_ROUTE_113, MAPSEC_ROUTE_111, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_ROUTE_119, MAPSEC_FORTREE_CITY, MAPSEC_ROUTE_120, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE},
     {MAPSEC_NONE, MAPSEC_ROUTE_114, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_MT_CHIMNEY, MAPSEC_MT_CHIMNEY, MAPSEC_ROUTE_111, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_ROUTE_119, MAPSEC_NONE, MAPSEC_ROUTE_120, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE},
     {MAPSEC_ROUTE_115, MAPSEC_ROUTE_114, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_MT_CHIMNEY, MAPSEC_MT_CHIMNEY, MAPSEC_ROUTE_111, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_ROUTE_119, MAPSEC_NONE, MAPSEC_ROUTE_120, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_SAFARI_ZONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE, MAPSEC_NONE},
diff --git a/src/daycare.c b/src/daycare.c
index 5ebe255b39..eb9a5fdeb0 100644
--- a/src/daycare.c
+++ b/src/daycare.c
@@ -830,7 +830,7 @@ void CreateEgg(struct Pokemon *mon, u16 species, bool8 setHotSpringsLocation)
     u8 metLevel;
     u16 ball;
     u8 language;
-    u8 metLocation;
+    metloc_u8_t metLocation;
     u8 isEgg;
 
     CreateMon(mon, species, EGG_HATCH_LEVEL, USE_RANDOM_IVS, FALSE, 0, OT_ID_PLAYER_ID, 0);
diff --git a/src/egg_hatch.c b/src/egg_hatch.c
index 5aa955d7e9..00a194ea70 100644
--- a/src/egg_hatch.c
+++ b/src/egg_hatch.c
@@ -362,7 +362,7 @@ static void AddHatchedMonToParty(u8 id)
     u8 name[POKEMON_NAME_LENGTH + 1];
     u16 ball;
     u16 metLevel;
-    u8 metLocation;
+    metloc_u8_t metLocation;
     struct Pokemon *mon = &gPlayerParty[id];
 
     CreateHatchedMon(mon, &gEnemyParty[0]);
diff --git a/src/frontier_pass.c b/src/frontier_pass.c
index a053badc62..fb6920e3cc 100644
--- a/src/frontier_pass.c
+++ b/src/frontier_pass.c
@@ -606,7 +606,9 @@ static void LeaveFrontierPass(void)
 
 static u32 AllocateFrontierPassData(MainCallback callback)
 {
-    u8 i;
+    // This variable is a MAPSEC initially, but is recycled as a 
+    // bare integer near the end of the function.
+    mapsec_u8_t i;
 
     if (sPassData != NULL)
         return ERR_ALREADY_DONE;
diff --git a/src/landmark.c b/src/landmark.c
index d2bb3c4105..a8cf6c1233 100644
--- a/src/landmark.c
+++ b/src/landmark.c
@@ -10,7 +10,7 @@ struct Landmark
 
 struct LandmarkList
 {
-    u8 mapSection;
+    mapsec_u8_t mapSection;
     u8 id;
     const struct Landmark *const *landmarks;
 };
@@ -392,9 +392,9 @@ static const struct LandmarkList sLandmarkLists[] =
     {MAPSEC_NONE, 0, NULL},
 };
 
-static const struct Landmark *const *GetLandmarks(u8 mapSection, u8 id);
+static const struct Landmark *const *GetLandmarks(mapsec_u8_t mapSection, u8 id);
 
-const u8 *GetLandmarkName(u8 mapSection, u8 id, u8 count)
+const u8 *GetLandmarkName(mapsec_u8_t mapSection, u8 id, u8 count)
 {
     const struct Landmark *const *landmarks = GetLandmarks(mapSection, id);
 
@@ -421,7 +421,7 @@ const u8 *GetLandmarkName(u8 mapSection, u8 id, u8 count)
     return (*landmarks)->name;
 }
 
-static const struct Landmark *const *GetLandmarks(u8 mapSection, u8 id)
+static const struct Landmark *const *GetLandmarks(mapsec_u8_t mapSection, u8 id)
 {
     u16 i = 0;
 
diff --git a/src/map_name_popup.c b/src/map_name_popup.c
index 4b30068f3c..665cccec3c 100644
--- a/src/map_name_popup.c
+++ b/src/map_name_popup.c
@@ -404,7 +404,7 @@ static void LoadMapNamePopUpWindowBg(void)
 {
     u8 popUpThemeId;
     u8 popupWindowId = GetMapNamePopUpWindowId();
-    u16 regionMapSectionId = gMapHeader.regionMapSectionId;
+    mapsec_u16_t regionMapSectionId = gMapHeader.regionMapSectionId;
 
     if (regionMapSectionId >= KANTO_MAPSEC_START)
     {
diff --git a/src/match_call.c b/src/match_call.c
index e6a141a1bc..41188f51bb 100644
--- a/src/match_call.c
+++ b/src/match_call.c
@@ -134,7 +134,7 @@ static u32 GetCurrentTotalMinutes(struct Time *);
 static u32 GetNumRegisteredTrainers(void);
 static u32 GetActiveMatchCallTrainerId(u32);
 static int GetTrainerMatchCallId(int);
-static u16 GetRematchTrainerLocation(int);
+static mapsec_u16_t GetRematchTrainerLocation(int);
 static bool32 TrainerIsEligibleForRematch(int);
 static void StartMatchCall(void);
 static void ExecuteMatchCall(u8);
@@ -1463,7 +1463,7 @@ static bool32 TrainerIsEligibleForRematch(int matchCallId)
     return gSaveBlock1Ptr->trainerRematches[matchCallId] > 0;
 }
 
-static u16 GetRematchTrainerLocation(int matchCallId)
+static mapsec_u16_t GetRematchTrainerLocation(int matchCallId)
 {
     const struct MapHeader *mapHeader = Overworld_GetMapHeaderByGroupAndId(gRematchTable[matchCallId].mapGroup, gRematchTable[matchCallId].mapNum);
     return mapHeader->regionMapSectionId;
diff --git a/src/overworld.c b/src/overworld.c
index 32c5eb0678..d1d3ddcd37 100644
--- a/src/overworld.c
+++ b/src/overworld.c
@@ -194,7 +194,7 @@ EWRAM_DATA struct WarpData gLastUsedWarp = {0};
 EWRAM_DATA static struct WarpData sWarpDestination = {0};  // new warp position
 EWRAM_DATA static struct WarpData sFixedDiveWarp = {0};
 EWRAM_DATA static struct WarpData sFixedHoleWarp = {0};
-EWRAM_DATA static u16 sLastMapSectionId = 0;
+EWRAM_DATA static mapsec_u16_t sLastMapSectionId = 0;
 EWRAM_DATA static struct InitialPlayerAvatarState sInitialPlayerAvatarState = {0};
 EWRAM_DATA static u16 sAmbientCrySpecies = 0;
 EWRAM_DATA static bool8 sIsAmbientCryWaterMon = FALSE;
@@ -1383,12 +1383,12 @@ bool8 IsMapTypeIndoors(u8 mapType)
         return FALSE;
 }
 
-u8 GetSavedWarpRegionMapSectionId(void)
+mapsec_u8_t GetSavedWarpRegionMapSectionId(void)
 {
     return Overworld_GetMapHeaderByGroupAndId(gSaveBlock1Ptr->dynamicWarp.mapGroup, gSaveBlock1Ptr->dynamicWarp.mapNum)->regionMapSectionId;
 }
 
-u8 GetCurrentRegionMapSectionId(void)
+mapsec_u8_t GetCurrentRegionMapSectionId(void)
 {
     return Overworld_GetMapHeaderByGroupAndId(gSaveBlock1Ptr->location.mapGroup, gSaveBlock1Ptr->location.mapNum)->regionMapSectionId;
 }
diff --git a/src/pokedex_area_screen.c b/src/pokedex_area_screen.c
index 8b0a845102..c3211ed454 100755
--- a/src/pokedex_area_screen.c
+++ b/src/pokedex_area_screen.c
@@ -58,7 +58,7 @@ struct OverworldArea
 {
     u8 mapGroup;
     u8 mapNum;
-    u16 regionMapSectionId;
+    mapsec_u16_t regionMapSectionId;
 };
 
 struct
@@ -79,7 +79,7 @@ struct
     /*0x61C*/ u16 areaShadeBldArgHi;
     /*0x61E*/ bool8 showingMarkers;
     /*0x61F*/ u8 markerFlashCounter;
-    /*0x620*/ u16 specialAreaRegionMapSectionIds[MAX_AREA_MARKERS];
+    /*0x620*/ mapsec_u16_t specialAreaRegionMapSectionIds[MAX_AREA_MARKERS];
     /*0x660*/ struct Sprite *areaMarkerSprites[MAX_AREA_MARKERS];
     /*0x6E0*/ u16 numAreaMarkerSprites;
     /*0x6E2*/ u16 alteringCaveCounter;
@@ -95,7 +95,7 @@ static void FindMapsWithMon(u16);
 static void BuildAreaGlowTilemap(void);
 static void SetAreaHasMon(u16, u16);
 static void SetSpecialMapHasMon(u16, u16);
-static u16 GetRegionMapSectionId(u8, u8);
+static mapsec_u16_t GetRegionMapSectionId(u8, u8);
 static bool8 MapHasSpecies(const struct WildPokemonHeader *, u16);
 static bool8 MonListHasSpecies(const struct WildPokemonInfo *, u16, u16);
 static void DoAreaGlow(void);
@@ -112,7 +112,7 @@ static const u32 sAreaGlow_Gfx[] = INCBIN_U32("graphics/pokedex/area_glow.4bpp.l
 
 static const u16 sSpeciesHiddenFromAreaScreen[] = { SPECIES_WYNAUT };
 
-static const u16 sMovingRegionMapSections[3] =
+static const mapsec_u16_t sMovingRegionMapSections[3] =
 {
     MAPSEC_MARINE_CAVE,
     MAPSEC_UNDERWATER_MARINE_CAVE,
@@ -125,7 +125,7 @@ static const u16 sFeebasData[][3] =
     {NUM_SPECIES}
 };
 
-static const u16 sLandmarkData[][2] =
+static const mapsec_u16_t sLandmarkData[][2] =
 {
     {MAPSEC_SKY_PILLAR,       FLAG_LANDMARK_SKY_PILLAR},
     {MAPSEC_SEAFLOOR_CAVERN,  FLAG_LANDMARK_SEAFLOOR_CAVERN},
@@ -336,7 +336,7 @@ static void SetSpecialMapHasMon(u16 mapGroup, u16 mapNum)
 
     if (sPokedexAreaScreen->numSpecialAreas < MAX_AREA_MARKERS)
     {
-        u16 regionMapSectionId = GetRegionMapSectionId(mapGroup, mapNum);
+        mapsec_u16_t regionMapSectionId = GetRegionMapSectionId(mapGroup, mapNum);
         if (regionMapSectionId < MAPSEC_NONE)
         {
             // Don't highlight the area if it's a moving area (Marine/Terra Cave)
@@ -370,7 +370,7 @@ static void SetSpecialMapHasMon(u16 mapGroup, u16 mapNum)
     }
 }
 
-static u16 GetRegionMapSectionId(u8 mapGroup, u8 mapNum)
+static mapsec_u16_t GetRegionMapSectionId(u8 mapGroup, u8 mapNum)
 {
     return Overworld_GetMapHeaderByGroupAndId(mapGroup, mapNum)->regionMapSectionId;
 }
@@ -710,7 +710,7 @@ static void CreateAreaMarkerSprites(void)
     static s16 x;
     static s16 y;
     static s16 i;
-    static s16 mapSecId;
+    static mapsec_s16_t mapSecId;
     static s16 numSprites;
 
     LoadSpriteSheet(&sAreaMarkerSpriteSheet);
diff --git a/src/pokemon.c b/src/pokemon.c
index a06693dc8a..7283e0e456 100644
--- a/src/pokemon.c
+++ b/src/pokemon.c
@@ -4074,6 +4074,19 @@ u32 GetBoxMonData2(struct BoxPokemon *boxMon, s32 field) __attribute__((alias("G
 #define SET8(lhs) (lhs) = *data
 #define SET16(lhs) (lhs) = data[0] + (data[1] << 8)
 #define SET32(lhs) (lhs) = data[0] + (data[1] << 8) + (data[2] << 16) + (data[3] << 24)
+//
+// Prefer SET_BY_WIDTH for fields whose types might be extended (e.g. 
+// anything whose typedef is in gametypes.h).
+//
+#define SET_BY_WIDTH(lhs) \
+    do { \
+       if (sizeof(lhs) == 1) \
+          SET8(lhs); \
+       else if (sizeof(lhs) == 2) \
+          SET16(lhs); \
+       else if (sizeof(lhs) == 4) \
+          SET32(lhs); \
+   } while (0)
 
 void SetMonData(struct Pokemon *mon, s32 field, const void *dataArg)
 {
@@ -4263,7 +4276,7 @@ void SetBoxMonData(struct BoxPokemon *boxMon, s32 field, const void *dataArg)
         SET8(substruct3->pokerus);
         break;
     case MON_DATA_MET_LOCATION:
-        SET8(substruct3->metLocation);
+        SET_BY_WIDTH(substruct3->metLocation);
         break;
     case MON_DATA_MET_LEVEL:
     {
diff --git a/src/pokemon_summary_screen.c b/src/pokemon_summary_screen.c
index a77df48327..734d925358 100644
--- a/src/pokemon_summary_screen.c
+++ b/src/pokemon_summary_screen.c
@@ -143,7 +143,7 @@ static EWRAM_DATA struct PokemonSummaryScreenData
         u8 ribbonCount; // 0x6
         u8 ailment; // 0x7
         u8 abilityNum; // 0x8
-        u8 metLocation; // 0x9
+        metloc_u8_t metLocation; // 0x9
         u8 metLevel; // 0xA
         u8 metGame; // 0xB
         u32 pid; // 0xC
diff --git a/src/pokenav_match_call_data.c b/src/pokenav_match_call_data.c
index 1c3cf9aeeb..3f47550f17 100644
--- a/src/pokenav_match_call_data.c
+++ b/src/pokenav_match_call_data.c
@@ -34,13 +34,13 @@ typedef struct MatchCallTextDataStruct {
 
 struct MatchCallStructCommon {
     u8 type;
-    u8 mapSec;
+    mapsec_u8_t mapSec;
     u16 flag;
 };
 
 struct MatchCallStructNPC {
     u8 type;
-    u8 mapSec;
+    mapsec_u8_t mapSec;
     u16 flag;
     const u8 *desc;
     const u8 *name;
@@ -50,7 +50,7 @@ struct MatchCallStructNPC {
 // Shared by MC_TYPE_TRAINER and MC_TYPE_LEADER
 struct MatchCallStructTrainer {
     u8 type;
-    u8 mapSec;
+    mapsec_u8_t mapSec;
     u16 flag;
     u16 rematchTableIdx;
     const u8 *desc;
@@ -60,12 +60,12 @@ struct MatchCallStructTrainer {
 
 struct MatchCallLocationOverride {
     u16 flag;
-    u8 mapSec;
+    mapsec_u8_t mapSec;
 };
 
 struct MatchCallWally {
     u8 type;
-    u8 mapSec;
+    mapsec_u8_t mapSec;
     u16 flag;
     u16 rematchTableIdx;
     const u8 *desc;
@@ -75,7 +75,7 @@ struct MatchCallWally {
 
 struct MatchCallBirch {
     u8 type;
-    u8 mapSec;
+    mapsec_u8_t mapSec;
     u16 flag;
     const u8 *desc;
     const u8 *name;
@@ -117,11 +117,11 @@ static bool32 MatchCall_GetEnabled_Wally(match_call_t);
 static bool32 MatchCall_GetEnabled_Birch(match_call_t);
 static bool32 MatchCall_GetEnabled_Rival(match_call_t);
 
-static u8 MatchCall_GetMapSec_NPC(match_call_t);
-static u8 MatchCall_GetMapSec_Trainer(match_call_t);
-static u8 MatchCall_GetMapSec_Wally(match_call_t);
-static u8 MatchCall_GetMapSec_Birch(match_call_t);
-static u8 MatchCall_GetMapSec_Rival(match_call_t);
+static mapsec_u8_t MatchCall_GetMapSec_NPC(match_call_t);
+static mapsec_u8_t MatchCall_GetMapSec_Trainer(match_call_t);
+static mapsec_u8_t MatchCall_GetMapSec_Wally(match_call_t);
+static mapsec_u8_t MatchCall_GetMapSec_Birch(match_call_t);
+static mapsec_u8_t MatchCall_GetMapSec_Rival(match_call_t);
 
 static bool32 MatchCall_IsRematchable_NPC(match_call_t);
 static bool32 MatchCall_IsRematchable_Trainer(match_call_t);
@@ -609,7 +609,7 @@ static bool32 (*const sMatchCallGetEnabledFuncs[])(match_call_t) = {
     MatchCall_GetEnabled_Birch
 };
 
-static u8 (*const sMatchCallGetMapSecFuncs[])(match_call_t) = {
+static mapsec_u8_t (*const sMatchCallGetMapSecFuncs[])(match_call_t) = {
     MatchCall_GetMapSec_NPC,
     MatchCall_GetMapSec_Trainer,
     MatchCall_GetMapSec_Wally,
@@ -779,7 +779,7 @@ static bool32 MatchCall_GetEnabled_Birch(match_call_t matchCall)
     return FlagGet(matchCall.birch->flag);
 }
 
-u8 MatchCall_GetMapSec(u32 idx)
+mapsec_u8_t MatchCall_GetMapSec(u32 idx)
 {
     match_call_t matchCall;
     u32 i;
@@ -791,17 +791,17 @@ u8 MatchCall_GetMapSec(u32 idx)
     return sMatchCallGetMapSecFuncs[i](matchCall);
 }
 
-static u8 MatchCall_GetMapSec_NPC(match_call_t matchCall)
+static mapsec_u8_t MatchCall_GetMapSec_NPC(match_call_t matchCall)
 {
     return matchCall.npc->mapSec;
 }
 
-static u8 MatchCall_GetMapSec_Trainer(match_call_t matchCall)
+static mapsec_u8_t MatchCall_GetMapSec_Trainer(match_call_t matchCall)
 {
     return matchCall.trainer->mapSec;
 }
 
-static u8 MatchCall_GetMapSec_Wally(match_call_t matchCall)
+static mapsec_u8_t MatchCall_GetMapSec_Wally(match_call_t matchCall)
 {
     s32 i;
 
@@ -813,12 +813,12 @@ static u8 MatchCall_GetMapSec_Wally(match_call_t matchCall)
     return matchCall.wally->locationData[i].mapSec;
 }
 
-static u8 MatchCall_GetMapSec_Rival(match_call_t matchCall)
+static mapsec_u8_t MatchCall_GetMapSec_Rival(match_call_t matchCall)
 {
     return MAPSEC_NONE;
 }
 
-static u8 MatchCall_GetMapSec_Birch(match_call_t matchCall)
+static mapsec_u8_t MatchCall_GetMapSec_Birch(match_call_t matchCall)
 {
     return MAPSEC_NONE;
 }
diff --git a/src/pokenav_match_call_gfx.c b/src/pokenav_match_call_gfx.c
index 4271e2ff23..dde725d8bd 100755
--- a/src/pokenav_match_call_gfx.c
+++ b/src/pokenav_match_call_gfx.c
@@ -1022,7 +1022,7 @@ static void PrintMatchCallLocation(struct Pokenav_MatchCallGfx *gfx, int delta)
     u8 mapName[32];
     int x;
     int index = PokenavList_GetSelectedIndex() + delta;
-    int mapSec = GetMatchCallMapSec(index);
+    mapsec_s32_t mapSec = GetMatchCallMapSec(index);
     if (mapSec != MAPSEC_NONE)
         GetMapName(mapName, mapSec, 0);
     else
diff --git a/src/pokenav_match_call_list.c b/src/pokenav_match_call_list.c
index fb89e33e0b..8f8fc6b4c2 100755
--- a/src/pokenav_match_call_list.c
+++ b/src/pokenav_match_call_list.c
@@ -306,7 +306,7 @@ struct PokenavMatchCallEntry *GetMatchCallList(void)
     return state->matchCallEntries;
 }
 
-u16 GetMatchCallMapSec(int index)
+mapsec_u16_t GetMatchCallMapSec(int index)
 {
     struct Pokenav_MatchCallMenu *state = GetSubstructPtr(POKENAV_SUBSTRUCT_MATCH_CALL_MAIN);
     return state->matchCallEntries[index].mapSec;
@@ -424,7 +424,7 @@ void BufferMatchCallNameAndDesc(struct PokenavMatchCallEntry *matchCallEntry, u8
     }
 }
 
-u8 GetMatchTableMapSectionId(int rematchIndex)
+mapsec_u8_t GetMatchTableMapSectionId(int rematchIndex)
 {
     int mapGroup = gRematchTable[rematchIndex].mapGroup;
     int mapNum = gRematchTable[rematchIndex].mapNum;
diff --git a/src/pokenav_region_map.c b/src/pokenav_region_map.c
index f81ff42966..8475bbacb8 100755
--- a/src/pokenav_region_map.c
+++ b/src/pokenav_region_map.c
@@ -41,7 +41,7 @@ struct Pokenav_RegionMapGfx
 
 struct CityMapEntry
 {
-    u16 mapSecId;
+    mapsec_u16_t mapSecId;
     u16 index;
     const u32 *tilemap;
 };
@@ -63,8 +63,8 @@ static bool32 IsDma3ManagerBusyWithBgCopy_(struct Pokenav_RegionMapGfx *);
 static void ChangeBgYForZoom(bool32);
 static bool32 IsChangeBgYForZoomActive(void);
 static void CreateCityZoomTextSprites(void);
-static void DrawCityMap(struct Pokenav_RegionMapGfx *, int, int);
-static void PrintLandmarkNames(struct Pokenav_RegionMapGfx *, int, int);
+static void DrawCityMap(struct Pokenav_RegionMapGfx *, mapsec_s32_t, int);
+static void PrintLandmarkNames(struct Pokenav_RegionMapGfx *, mapsec_s32_t, int);
 static void SetCityZoomTextInvisibility(bool32);
 static void Task_ChangeBgYForZoom(u8 taskId);
 static void UpdateCityZoomTextPosition(void);
@@ -634,7 +634,7 @@ static u32 LoopedTask_DecompressCityMaps(s32 taskState)
     return LT_FINISH;
 }
 
-static void DrawCityMap(struct Pokenav_RegionMapGfx *state, int mapSecId, int pos)
+static void DrawCityMap(struct Pokenav_RegionMapGfx *state, mapsec_s32_t mapSecId, int pos)
 {
     int i;
     for (i = 0; i < NUM_CITY_MAPS && (sPokenavCityMaps[i].mapSecId != mapSecId || sPokenavCityMaps[i].index != pos); i++)
@@ -647,7 +647,7 @@ static void DrawCityMap(struct Pokenav_RegionMapGfx *state, int mapSecId, int po
     CopyToBgTilemapBufferRect(1, state->cityZoomPics[i], 18, 6, 10, 10);
 }
 
-static void PrintLandmarkNames(struct Pokenav_RegionMapGfx *state, int mapSecId, int pos)
+static void PrintLandmarkNames(struct Pokenav_RegionMapGfx *state, mapsec_s32_t mapSecId, int pos)
 {
     int i = 0;
     while (1)
diff --git a/src/region_map.c b/src/region_map.c
index 22a3f567b4..a418b7fe4f 100644
--- a/src/region_map.c
+++ b/src/region_map.c
@@ -63,7 +63,7 @@ enum {
 struct MultiNameFlyDest
 {
     const u8 *const *name;
-    u16 mapSecId;
+    mapsec_u16_t mapSecId;
     u16 flag;
 };
 
@@ -72,7 +72,7 @@ static EWRAM_DATA struct RegionMap *sRegionMap = NULL;
 static EWRAM_DATA struct {
     void (*callback)(void);
     u16 state;
-    u16 mapSecId;
+    mapsec_u16_t mapSecId;
     struct RegionMap regionMap;
     u8 tileBuffer[0x1c0];
     u8 nameBuffer[0x26]; // never read
@@ -86,15 +86,15 @@ static u8 MoveRegionMapCursor_Full(void);
 static u8 ProcessRegionMapInput_Zoomed(void);
 static u8 MoveRegionMapCursor_Zoomed(void);
 static void CalcZoomScrollParams(s16 scrollX, s16 scrollY, s16 c, s16 d, u16 e, u16 f, u8 rotation);
-static u16 GetMapSecIdAt(u16 x, u16 y);
+static mapsec_u16_t GetMapSecIdAt(u16 x, u16 y);
 static void RegionMap_SetBG2XAndBG2Y(s16 x, s16 y);
 static void InitMapBasedOnPlayerLocation(void);
 static void RegionMap_InitializeStateBasedOnSSTidalLocation(void);
-static u8 GetMapsecType(u16 mapSecId);
-static u16 CorrectSpecialMapSecId_Internal(u16 mapSecId);
-static u16 GetTerraOrMarineCaveMapSecId(void);
+static u8 GetMapsecType(mapsec_u16_t mapSecId);
+static mapsec_u16_t CorrectSpecialMapSecId_Internal(mapsec_u16_t mapSecId);
+static mapsec_u16_t GetTerraOrMarineCaveMapSecId(void);
 static void GetMarineCaveCoords(u16 *x, u16 *y);
-static bool32 IsPlayerInAquaHideout(u8 mapSecId);
+static bool32 IsPlayerInAquaHideout(mapsec_u8_t mapSecId);
 static void GetPositionOfCursorWithinMapSec(void);
 static bool8 RegionMap_IsMapSecIdInNextRow(u16 y);
 static void SpriteCB_CursorMapFull(struct Sprite *sprite);
@@ -130,7 +130,7 @@ static const u8 sRegionMapPlayerIcon_MayGfx[] = INCBIN_U8("graphics/pokenav/regi
 #include "data/region_map/region_map_layout.h"
 #include "data/region_map/region_map_entries.h"
 
-static const u16 sRegionMap_SpecialPlaceLocations[][2] =
+static const mapsec_u16_t sRegionMap_SpecialPlaceLocations[][2] =
 {
     {MAPSEC_UNDERWATER_105,             MAPSEC_ROUTE_105},
     {MAPSEC_UNDERWATER_124,             MAPSEC_ROUTE_124},
@@ -162,14 +162,14 @@ static const u16 sRegionMap_SpecialPlaceLocations[][2] =
     {MAPSEC_NONE,                       MAPSEC_NONE}
 };
 
-static const u16 sMarineCaveMapSecIds[] =
+static const mapsec_u16_t sMarineCaveMapSecIds[] =
 {
     MAPSEC_MARINE_CAVE,
     MAPSEC_UNDERWATER_MARINE_CAVE,
     MAPSEC_UNDERWATER_MARINE_CAVE
 };
 
-static const u16 sTerraOrMarineCaveMapSecIds[ABNORMAL_WEATHER_LOCATIONS] =
+static const mapsec_u16_t sTerraOrMarineCaveMapSecIds[ABNORMAL_WEATHER_LOCATIONS] =
 {
     [ABNORMAL_WEATHER_ROUTE_114_NORTH - 1] = MAPSEC_ROUTE_114,
     [ABNORMAL_WEATHER_ROUTE_114_SOUTH - 1] = MAPSEC_ROUTE_114,
@@ -203,7 +203,7 @@ static const struct UCoords16 sMarineCaveLocationCoords[MARINE_CAVE_LOCATIONS] =
     [MARINE_CAVE_COORD(ROUTE_129_EAST)]  = {24, 10}
 };
 
-static const u8 sMapSecAquaHideoutOld[] =
+static const mapsec_u8_t sMapSecAquaHideoutOld[] =
 {
     MAPSEC_AQUA_HIDEOUT_OLD
 };
@@ -273,7 +273,7 @@ static const union AnimCmd *const sRegionMapPlayerIconAnimTable[] =
 };
 
 // Event islands that don't appear on map. (Southern Island does)
-static const u8 sMapSecIdsOffMap[] =
+static const mapsec_u8_t sMapSecIdsOffMap[] =
 {
     MAPSEC_BIRTH_ISLAND,
     MAPSEC_FARAWAY_ISLAND,
@@ -421,7 +421,7 @@ static const struct SpritePalette sFlyTargetIconsSpritePalette =
     .tag = TAG_FLY_ICON
 };
 
-static const u16 sRedOutlineFlyDestinations[][2] =
+static const mapsec_u16_t sRedOutlineFlyDestinations[][2] =
 {
     {
         FLAG_LANDMARK_BATTLE_FRONTIER,
@@ -690,7 +690,7 @@ static u8 ProcessRegionMapInput_Full(void)
 
 static u8 MoveRegionMapCursor_Full(void)
 {
-    u16 mapSecId;
+    mapsec_u16_t mapSecId;
 
     if (sRegionMap->cursorMovementFrameCounter != 0)
         return MAP_INPUT_MOVE_CONT;
@@ -771,7 +771,7 @@ static u8 MoveRegionMapCursor_Zoomed(void)
 {
     u16 x;
     u16 y;
-    u16 mapSecId;
+    mapsec_u16_t mapSecId;
 
     sRegionMap->scrollY += sRegionMap->zoomedCursorDeltaY;
     sRegionMap->scrollX += sRegionMap->zoomedCursorDeltaX;
@@ -954,7 +954,7 @@ void PokedexAreaScreen_UpdateRegionMapVariablesAndVideoRegs(s16 x, s16 y)
     }
 }
 
-static u16 GetMapSecIdAt(u16 x, u16 y)
+static mapsec_u16_t GetMapSecIdAt(u16 x, u16 y)
 {
     if (y < MAPCURSOR_Y_MIN || y > MAPCURSOR_Y_MAX || x < MAPCURSOR_X_MIN || x > MAPCURSOR_X_MAX)
     {
@@ -1172,7 +1172,7 @@ static void RegionMap_InitializeStateBasedOnSSTidalLocation(void)
     sRegionMap->cursorPosY = gRegionMapEntries[sRegionMap->mapSecId].y + y + MAPCURSOR_Y_MIN;
 }
 
-static u8 GetMapsecType(u16 mapSecId)
+static u8 GetMapsecType(mapsec_u16_t mapSecId)
 {
     switch (mapSecId)
     {
@@ -1219,12 +1219,12 @@ static u8 GetMapsecType(u16 mapSecId)
     }
 }
 
-u16 GetRegionMapSecIdAt(u16 x, u16 y)
+mapsec_u16_t GetRegionMapSecIdAt(u16 x, u16 y)
 {
     return GetMapSecIdAt(x, y);
 }
 
-static u16 CorrectSpecialMapSecId_Internal(u16 mapSecId)
+static mapsec_u16_t CorrectSpecialMapSecId_Internal(mapsec_u16_t mapSecId)
 {
     u32 i;
 
@@ -1245,7 +1245,7 @@ static u16 CorrectSpecialMapSecId_Internal(u16 mapSecId)
     return mapSecId;
 }
 
-static u16 GetTerraOrMarineCaveMapSecId(void)
+static mapsec_u16_t GetTerraOrMarineCaveMapSecId(void)
 {
     s16 idx;
 
@@ -1274,7 +1274,7 @@ static void GetMarineCaveCoords(u16 *x, u16 *y)
 
 // Probably meant to be an "IsPlayerInIndoorDungeon" function, but in practice it only has the one mapsec
 // Additionally, because the mapsec doesnt exist in Emerald, this function always returns FALSE
-static bool32 IsPlayerInAquaHideout(u8 mapSecId)
+static bool32 IsPlayerInAquaHideout(mapsec_u8_t mapSecId)
 {
     u32 i;
 
@@ -1286,7 +1286,7 @@ static bool32 IsPlayerInAquaHideout(u8 mapSecId)
     return FALSE;
 }
 
-u16 CorrectSpecialMapSecId(u16 mapSecId)
+mapsec_u16_t CorrectSpecialMapSecId(mapsec_u16_t mapSecId)
 {
     return CorrectSpecialMapSecId_Internal(mapSecId);
 }
@@ -1565,7 +1565,7 @@ void TrySetPlayerIconBlink(void)
 #undef sVisible
 #undef sTimer
 
-u8 *GetMapName(u8 *dest, u16 regionMapId, u16 padLength)
+u8 *GetMapName(u8 *dest, mapsec_u16_t regionMapId, u16 padLength)
 {
     u8 *str;
     u16 i;
@@ -1598,7 +1598,7 @@ u8 *GetMapName(u8 *dest, u16 regionMapId, u16 padLength)
 }
 
 // TODO: probably needs a better name
-u8 *GetMapNameGeneric(u8 *dest, u16 mapSecId)
+u8 *GetMapNameGeneric(u8 *dest, mapsec_u16_t mapSecId)
 {
     switch (mapSecId)
     {
@@ -1611,7 +1611,7 @@ u8 *GetMapNameGeneric(u8 *dest, u16 mapSecId)
     }
 }
 
-u8 *GetMapNameHandleAquaHideout(u8 *dest, u16 mapSecId)
+u8 *GetMapNameHandleAquaHideout(u8 *dest, mapsec_u16_t mapSecId)
 {
     if (mapSecId == MAPSEC_AQUA_HIDEOUT_OLD)
         return StringCopy(dest, gText_Hideout);
@@ -1619,7 +1619,7 @@ u8 *GetMapNameHandleAquaHideout(u8 *dest, u16 mapSecId)
         return GetMapNameGeneric(dest, mapSecId);
 }
 
-static void GetMapSecDimensions(u16 mapSecId, u16 *x, u16 *y, u16 *width, u16 *height)
+static void GetMapSecDimensions(mapsec_u16_t mapSecId, u16 *x, u16 *y, u16 *width, u16 *height)
 {
     *x = gRegionMapEntries[mapSecId].x;
     *y = gRegionMapEntries[mapSecId].y;
@@ -1632,7 +1632,7 @@ bool8 IsRegionMapZoomed(void)
     return sRegionMap->zoomed;
 }
 
-bool32 IsEventIslandMapSecId(u8 mapSecId)
+bool32 IsEventIslandMapSecId(mapsec_u8_t mapSecId)
 {
     u32 i;
 
@@ -1839,7 +1839,7 @@ static void LoadFlyDestIcons(void)
 static void CreateFlyDestIcons(void)
 {
     u16 canFlyFlag;
-    u16 mapSecId;
+    mapsec_u16_t mapSecId;
     u16 x;
     u16 y;
     u16 width;
@@ -1887,7 +1887,7 @@ static void TryCreateRedOutlineFlyDestIcons(void)
     u16 y;
     u16 width;
     u16 height;
-    u16 mapSecId;
+    mapsec_u16_t mapSecId;
     u8 spriteId;
 
     for (i = 0; sRedOutlineFlyDestinations[i][1] != MAPSEC_NONE; i++)
diff --git a/src/trade.c b/src/trade.c
index e07e418d1a..e9c2984087 100644
--- a/src/trade.c
+++ b/src/trade.c
@@ -4552,7 +4552,7 @@ static void CreateInGameTradePokemonInternal(u8 whichPlayerMon, u8 whichInGameTr
     u8 level = GetMonData(&gPlayerParty[whichPlayerMon], MON_DATA_LEVEL);
 
     struct Mail mail;
-    u8 metLocation = METLOC_IN_GAME_TRADE;
+    metloc_u8_t metLocation = METLOC_IN_GAME_TRADE;
     u8 mailNum;
     struct Pokemon *pokemon = &gEnemyParty[0];