diff --git a/README.md b/README.md
index ac8c1e32..befd998c 100644
--- a/README.md
+++ b/README.md
@@ -2,8 +2,12 @@
 
 These fundamental modules are helpful when I am playing OpenSCAD.
 
-## 2D modules
+## 2D
 - [line2d](https://openhome.cc/eGossip/OpenSCAD/lib-line2d.html)
 - [polyline2d](https://openhome.cc/eGossip/OpenSCAD/lib-polyline2d.html)
 - [circular_sector](https://openhome.cc/eGossip/OpenSCAD/lib-circular_sector.html)
 - [arc](https://openhome.cc/eGossip/OpenSCAD/lib-arc.html)
+
+## Transformations
+- [hollow_out](https://openhome.cc/eGossip/OpenSCAD/lib-hollow_out.html)
+