diff --git a/synapse/storage/events.py b/synapse/storage/events.py
index 93ac80ee4..5ebef98c4 100644
--- a/synapse/storage/events.py
+++ b/synapse/storage/events.py
@@ -1931,7 +1931,8 @@ class EventsStore(EventsWorkerStore):
         logger.info("[purge] Finding new backward extremities")
 
         # We calculate the new entries for the backward extremeties by finding
-        # all events that point to events that are to be purged
+        # events to be purged that are pointed to by events we're not going to
+        # purge.
         txn.execute(
             "SELECT DISTINCT e.event_id FROM events_to_purge AS e"
             " INNER JOIN event_edges AS ed ON e.event_id = ed.prev_event_id"