diff --git a/src/structs.h b/src/structs.h
index 32c35b7..fe4704a 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -1309,6 +1309,9 @@
 struct mapblock
 {
     mapblock_T	*m_next;	// next mapblock in list
+    mapblock_T	*m_alt;		// pointer to mapblock of the same mapping
+				// with an alternative form of m_keys, or NULL
+				// if there is no such mapblock
     char_u	*m_keys;	// mapped from, lhs
     char_u	*m_str;		// mapped to, rhs
     char_u	*m_orig_str;	// rhs as entered by the user