diff --git a/doc/planar_canonical_ordering.html b/doc/planar_canonical_ordering.html
index c5311d17..035475fd 100644
--- a/doc/planar_canonical_ordering.html
+++ b/doc/planar_canonical_ordering.html
@@ -82,6 +82,16 @@ IN: <tt>Graph&amp; g</tt>
 <blockquote>
 An undirected graph. The graph type must be a model of 
 <a href="VertexAndEdgeListGraph.html">VertexAndEdgeListGraph</a>.
+The graph must:
+<ul>
+<li>Be maximal planar.</li>
+<li>Have at least two vertices.</li>
+<li>Have the edge <i>{v<sub>0</sub>, v<sub>1</sub>}</i> on its outer face,
+where <i>v<sub>0</sub></i> is <tt>*vertices(g).first</tt> and
+<i>v<sub>1</sub></i> is the first element of
+<tt>adjacent_vertices(</tt><i>v<sub>0</sub></i><tt>, g)</tt> distinct from
+<i>v<sub>0</sub></i>.</li>
+</ul>
 </blockquote>
 
 IN: <tt>PlanarEmbedding</tt>