diff --git a/readme.md b/readme.md
index 8976a4a..b635939 100644
--- a/readme.md
+++ b/readme.md
@@ -165,6 +165,8 @@ exposing enough information to make a battery box.
 Models timing belt running over toothed or smooth pulleys and calculates an accurate length.
 Only models 2D paths, so not core XY!
 
+To make the back of the belt run against a smooth pulley on the outside of the loop specify a negative pitch radius.
+
 By default the path is a closed loop but a gap length and position can be specified to make open loops.
 
 Individual teeth are not drawn, instead they are represented by a lighter colour.
diff --git a/vitamins/belt.scad b/vitamins/belt.scad
index 37ad1d5..48b9da5 100644
--- a/vitamins/belt.scad
+++ b/vitamins/belt.scad
@@ -21,6 +21,8 @@
 //! Models timing belt running over toothed or smooth pulleys and calculates an accurate length.
 //! Only models 2D paths, so not core XY!
 //!
+//! To make the back of the belt run against a smooth pulley on the outside of the loop specify a negative pitch radius.
+//!
 //! By default the path is a closed loop but a gap length and position can be specified to make open loops.
 //!
 //! Individual teeth are not drawn, instead they are represented by a lighter colour.