diff --git a/wiringPi/wiringPi.h b/wiringPi/wiringPi.h
index 6acb57e..7ed9078 100644
--- a/wiringPi/wiringPi.h
+++ b/wiringPi/wiringPi.h
@@ -86,6 +86,7 @@
 
 // Pi model types and version numbers
 //	Intended for the GPIO program Use at your own risk.
+// https://www.raspberrypi.com/documentation/computers/raspberry-pi.html#new-style-revision-codes
 
 #define	PI_MODEL_A		 0
 #define	PI_MODEL_B		 1