diff --git a/math/computational_geometry/notes.page b/math/computational_geometry/notes.page
index 656fd1d..881a0ce 100644
--- a/math/computational_geometry/notes.page
+++ b/math/computational_geometry/notes.page
@@ -26,3 +26,9 @@ are *vertices*. The set of vertices and edges is the *boundary*.
 :The boundary $\partial P$ of a polygon $P$ partitions the plane into
 two parts.  In particular, the two components of $\Bbb{R}^2\setminus
 \partial P$ are the bounded interior and the unbounded exterior.
+
+A point $x$ is *interior* if a ray through it in a fixed direction not
+parallel to an edge passes through an odd number of edges.
+
+A point $x$ is *exterior$ if a ray through it ... etc ... through an
+even number of edges.