diff --git a/calendar.go b/calendar.go
index c17aae5..1cd88ac 100644
--- a/calendar.go
+++ b/calendar.go
@@ -60,6 +60,7 @@ const (
 	ComponentPropertyPercentComplete = ComponentProperty(PropertyPercentComplete)
 	ComponentPropertyTzid            = ComponentProperty(PropertyTzid)
 	ComponentPropertyComment         = ComponentProperty(PropertyComment)
+	ComponentPropertyRelatedTo       = ComponentProperty(PropertyRelatedTo)
 )
 
 type Property string