diff --git a/diamond-model/machinetag.json b/diamond-model/machinetag.json
index 61f5e72..beeec6c 100644
--- a/diamond-model/machinetag.json
+++ b/diamond-model/machinetag.json
@@ -3,6 +3,7 @@
   "expanded": "Diamond Model for Intrusion Analysis",
   "description": "The Diamond Model for Intrusion Analysis, a phase-based model developed by Lockheed Martin, aims to help categorise and identify the stage of an attack.",
   "version": 1,
+  "ref": ["http://www.activeresponse.org/wp-content/uploads/2013/07/diamond.pdf"],
   "predicates": [
     {
       "value": "Adversary",