diff --git a/lib/matplotlib/contour.py b/lib/matplotlib/contour.py
index 9f1307c44791..7e76aa39f8df 100644
--- a/lib/matplotlib/contour.py
+++ b/lib/matplotlib/contour.py
@@ -1593,6 +1593,8 @@ def _initialize_x_y(self, z):
     that ``len(X) == N`` is the number of columns in *Z* and
     ``len(Y) == M`` is the number of rows in *Z*.
 
+    *X* and *Y* must both be ordered monotonically.
+
     If not given, they are assumed to be integer indices, i.e.
     ``X = range(N)``, ``Y = range(M)``.