diff --git a/readme.md b/readme.md
index 00a59ba..c85d0bc 100644
--- a/readme.md
+++ b/readme.md
@@ -2904,6 +2904,8 @@ These items are sysmtrical, so by default the origin is in the centre but it can
 ## Screws
 Machine screws and wood screws with various head styles.
 
+For an explanation of ```screw_polysink()``` see <https://hydraraptor.blogspot.com/2020/12/sinkholes.html>.
+
 
 [vitamins/screws.scad](vitamins/screws.scad) Object definitions.
 
diff --git a/vitamins/screw.scad b/vitamins/screw.scad
index 3cbb3ed..b833b59 100644
--- a/vitamins/screw.scad
+++ b/vitamins/screw.scad
@@ -19,6 +19,8 @@
 
 //
 //! Machine screws and wood screws with various head styles.
+//!
+//! For an explanation of ```screw_polysink()``` see <https://hydraraptor.blogspot.com/2020/12/sinkholes.html>.
 //
 include <../utils/core/core.scad>