diff --git a/supporting-docs/guides/2016-10-18-e2e_implementation.rst b/supporting-docs/guides/2016-10-18-e2e_implementation.rst
index a754f7c64..d0e1ca5bd 100644
--- a/supporting-docs/guides/2016-10-18-e2e_implementation.rst
+++ b/supporting-docs/guides/2016-10-18-e2e_implementation.rst
@@ -396,7 +396,8 @@ These events contain key data to allow decryption of other messages.
 They are sent to specific devices, so they appear in the ``to_device``
 section of the response to ``GET /_matrix/client/r0/sync``. They will
 also be encrypted, so will need decrypting as above before they can be
-seen.
+seen. (These events are generated by other clients - see `starting a megolm
+session`_).
 
 The event content will contain an 'algorithm' property, indicating the
 encryption algorithm the key data is to be used for. Currently, this