diff --git a/include/xkbsrv.h b/include/xkbsrv.h
index a19c8fb29..d58478543 100644
--- a/include/xkbsrv.h
+++ b/include/xkbsrv.h
@@ -301,6 +301,11 @@ extern _X_EXPORT const char *XkbBinDirectory;
 extern _X_EXPORT CARD32 xkbDebugFlags;
 
 #define	_XkbLibError(c,l,d)     /* Epoch fail */
+
+/* "a" is a "unique" numeric identifier that just defines which error
+ * code statement it is. _XkbErrCode2(4, foo) means "this is the 4th error
+ * statement in this function". lovely.
+ */
 #define	_XkbErrCode2(a,b) ((XID)((((unsigned int)(a))<<24)|((b)&0xffffff)))
 #define	_XkbErrCode3(a,b,c)	_XkbErrCode2(a,(((unsigned int)(b))<<16)|(c))
 #define	_XkbErrCode4(a,b,c,d) _XkbErrCode3(a,b,((((unsigned int)(c))<<8)|(d)))