diff --git a/src/hb-common.h b/src/hb-common.h
index 9546b21c6..a30587a08 100644
--- a/src/hb-common.h
+++ b/src/hb-common.h
@@ -74,6 +74,7 @@ typedef union _hb_var_int_t {
 typedef uint32_t hb_tag_t;
 
 #define HB_TAG(a,b,c,d) ((hb_tag_t)((((uint8_t)(a))<<24)|(((uint8_t)(b))<<16)|(((uint8_t)(c))<<8)|((uint8_t)(d))))
+#define HB_UNTAG(tag)   ((uint8_t)((tag)>>24)), ((uint8_t)((tag)>>16)), ((uint8_t)((tag)>>8)), ((uint8_t)(tag))
 
 #define HB_TAG_NONE HB_TAG(0,0,0,0)