diff --git a/README.md b/README.md
index 7cb75d1..c57cd9b 100644
--- a/README.md
+++ b/README.md
@@ -95,6 +95,10 @@ Take this `angle`, and return a number between 0 and 360 that represents it as a
 
 Set the ink with which the turtle draws to this `colour`, which should be a string representation of a colour known to CSS.
 
+#### (set-nib! *n*)
+
+Set the nib (width) of the pen to this `n`, which should be a number.
+
 #### (sin *angle*)
 
 Return the sine of this `angle`, considered to be expressed in degrees.