diff --git a/math/computational_geometry/notes.page b/math/computational_geometry/notes.page
index c2a1451..88ec5dc 100644
--- a/math/computational_geometry/notes.page
+++ b/math/computational_geometry/notes.page
@@ -32,3 +32,16 @@ parallel to an edge passes through an odd number of edges.
 
 A point $x$ is *exterior* if a ray through it ... etc ... passes
 through an even number of edges.
+
+These two facts form the basis of an algorithm for determining whether
+a point is inside a polygon.
+
+*diagonal*
+: A line segment of $P$ connecting two vertices and lying in the interior of
+$P$, not touching $\partial P$ except at endpoints. Two diagonals are
+*noncrossing* if they share no interior points.
+
+*triangulation*
+: A decomposition of $P$ into triangles by a maximal set of
+noncrossing diagonals. Maximal means that no more diagonals may be
+added without crossing.