diff --git a/README.md b/README.md
index 97943e5..0c2dcce 100644
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+++ b/README.md
@@ -12,6 +12,8 @@ Live at [n3xd.io](https://n3xd.io).
 
 ## The moving parts
 
+- **`app`** the CAD app itself. Sketches, features, assemblies, and the buttons
+  you actually click.
 - **`ocp`** hand-written bindings to the OpenCASCADE geometry kernel, so Python
   can talk to solids without the usual suffering.
 - **`assay`** the simulation engine. Mesh in, fields out. Tells you whether the