diff --git a/source/tutorial/create-a-unique-index.txt b/source/tutorial/create-a-unique-index.txt
index 0f20c360445..4ea91a4efd3 100644
--- a/source/tutorial/create-a-unique-index.txt
+++ b/source/tutorial/create-a-unique-index.txt
@@ -38,6 +38,15 @@ The :ref:`_id index <index-type-id>` is a unique index. In some
 situations you may consider using ``_id`` field itself for this kind
 of data rather than using a unique index on another field.
 
+The unicity constraint prevents two documents from the collection to
+have the same value, but it does not prevent one specific document to
+have a value duplicated. For instance, a unique index on ``a.b`` 
+will let the following document exist:
+
+.. code-block:: javascript
+
+   { "a" : [ { "b": 1 }, { "b" : 1 } ] }
+
 In many situations you will want to combine the ``unique`` constraint
 with the ``sparse`` option. When MongoDB indexes a field, if a
 document does not have a value for a field, the index entry for that