diff --git a/src/signs_type.h b/src/signs_type.h
--- a/src/signs_type.h
+++ b/src/signs_type.h
@@ -12,10 +12,11 @@
 #ifndef SIGNS_TYPE_H
 #define SIGNS_TYPE_H
 
+/** The type of the IDs of signs. */
 typedef uint16 SignID;
 struct Sign;
 
-static const SignID INVALID_SIGN = 0xFFFF;
+static const SignID INVALID_SIGN = 0xFFFF; ///< Sentinel for an invalid sign.
 
 static const uint MAX_LENGTH_SIGN_NAME_CHARS = 32; ///< The maximum length of a sign name in characters including '\0'