diff --git a/csa/_elementary.py b/csa/_elementary.py
index fdb22a6..70abac8 100644
--- a/csa/_elementary.py
+++ b/csa/_elementary.py
@@ -1,3 +1,15 @@
+"""
+Randomness and reproducibility
+------------------------------
+This module relies on Python's global random number generator.
+Random values are drawn from the current RNG state and the RNG
+is not re-seeded on each call.
+
+As a result, repeated CSA operations (e.g. random/block-style
+masks) with identical parameters may produce different
+realizations unless the user explicitly sets a random seed
+using ``random.seed(...)`` before invoking them.
+"""
 #
 #  This file is part of the Connection-Set Algebra (CSA).
 #  Copyright (C) 2010,2011,2012 Mikael Djurfeldt