diff --git a/spacy/syntax/transition_system.pyx b/spacy/syntax/transition_system.pyx
index b76c97566..fc84fc23a 100644
--- a/spacy/syntax/transition_system.pyx
+++ b/spacy/syntax/transition_system.pyx
@@ -178,6 +178,9 @@ cdef class TransitionSystem:
             self.c = <Transition*>self.mem.realloc(self.c, self._size * sizeof(self.c[0]))
         self.c[self.n_moves] = self.init_transition(self.n_moves, action, label_id)
         self.n_moves += 1
+        # Add the new (action, label) pair, making up a frequency for it if
+        # necessary. To preserve sort order, the frequency needs to be lower
+        # than previous frequencies.
         if self.labels.get(action, []):
             new_freq = min(self.labels[action].values())
         else: