diff --git a/code/__DEFINES/misc.dm b/code/__DEFINES/misc.dm
index 42095aba41..5fbe66808f 100644
--- a/code/__DEFINES/misc.dm
+++ b/code/__DEFINES/misc.dm
@@ -9,6 +9,9 @@
 #define TEXT_EAST			"[EAST]"
 #define TEXT_WEST			"[WEST]"
 
+/// Inverse direction, taking into account UP|DOWN if necessary.
+#define REVERSE_DIR(dir) ( ((dir & 85) << 1) | ((dir & 170) >> 1) )
+
 //These get to go at the top, because they're special
 //You can use these defines to get the typepath of the currently running proc/verb (yes procs + verbs are objects)
 /* eg:
diff --git a/code/modules/atmospherics/environmental/LINDA_system.dm b/code/modules/atmospherics/environmental/LINDA_system.dm
index 21dc6069a6..4f057ca9be 100644
--- a/code/modules/atmospherics/environmental/LINDA_system.dm
+++ b/code/modules/atmospherics/environmental/LINDA_system.dm
@@ -18,7 +18,7 @@
 
 /turf/open/CanAtmosPass(turf/T, vertical = FALSE)
 	var/dir = vertical? get_dir_multiz(src, T) : get_dir(src, T)
-	var/opp = dir_inverse_multiz(dir)
+	var/opp = REVERSE_DIR(dir)
 	var/R = FALSE
 	if(vertical && !(zAirOut(dir, T) && T.zAirIn(dir, src)))
 		R = TRUE
@@ -48,7 +48,7 @@
 	var/canvpass = CANVERTICALATMOSPASS(src, src)
 	for(var/direction in GLOB.cardinals_multiz)
 		var/turf/T = get_step_multiz(src, direction)
-		var/opp_dir = dir_inverse_multiz(direction)
+		var/opp_dir = REVERSE_DIR(direction)
 		if(!isopenturf(T))
 			continue
 		if(!(blocks_air || T.blocks_air) && ((direction & (UP|DOWN))? (canvpass && CANVERTICALATMOSPASS(T, src)) : (canpass && CANATMOSPASS(T, src))) )
diff --git a/code/modules/mapping/space_management/multiz_helpers.dm b/code/modules/mapping/space_management/multiz_helpers.dm
index 970c25ee9b..81c78cec8c 100644
--- a/code/modules/mapping/space_management/multiz_helpers.dm
+++ b/code/modules/mapping/space_management/multiz_helpers.dm
@@ -46,17 +46,4 @@
 
 /turf/proc/below()
 	return get_step_multiz(src, DOWN)
-
-/proc/dir_inverse_multiz(dir)
-	var/holder = dir & (UP|DOWN)
-	if((holder == NONE) || (holder == (UP|DOWN)))
-		return turn(dir, 180)
-	dir &= ~(UP|DOWN)
-	if(dir != 0)
-		dir = turn(dir, 180)
-	if(holder == UP)
-		holder = DOWN
-	else
-		holder = UP
-	dir |= holder
-	return dir
+	
\ No newline at end of file