diff --git a/fsl/data/atlases.py b/fsl/data/atlases.py
index 7920c962783bfbc01a14e4bc1aa0cdfc9aabff43..b47e4b6308f6b11b9cf90d5d4292f03bdd30f965 100644
--- a/fsl/data/atlases.py
+++ b/fsl/data/atlases.py
@@ -576,6 +576,10 @@ class AtlasDescription(object):
         ``ValueError`` is raised otherwise. If an invalid ``index`` or
         ``value`` is specified, an ``IndexError`` or ``KeyError`` will be
         raised.
+
+        .. note:: A 4D ``ProbabilisticAtlas`` may have more volumes than
+                  labels, and a 3D ``LabelAtlas`` may have more values
+                  than labels.
         """
         if (index is     None and value is     None) or \
            (index is not None and value is not None):