diff --git a/README.md b/README.md
index 1c5c105..aa77e61 100644
--- a/README.md
+++ b/README.md
@@ -9,6 +9,8 @@ Features include:
 Example FORTH files included for N-Queens and a simple graphical game, Snake.
 
 See a demonstration at: https://www.youtube.com/watch?v=ILMbia3-VZo
+
+Based on: https://www.openbookproject.net/py4fun/forth/forth.html
 # Installation
 Use the HP Connectivity Kit to transfer FORTH.hpprgm to a virtual or physical HP Prime calculator.