diff --git a/doc/changes/splitCyclic.txt b/doc/changes/splitCyclic.txt
index fc0ff4880f6ffa3c0ec73d62c35b4e412e56ec8b..814dfe0850cd3c4afe0857cbf24c7d1a34f687cc 100644
--- a/doc/changes/splitCyclic.txt
+++ b/doc/changes/splitCyclic.txt
@@ -104,6 +104,14 @@ type 'processorCyclic'.
 
 - processor patches use overlapping communication using a different message
 tag. This maps straight through into the MPI message tag.
+See processorCyclicPolyPatch::tag(). This needs to be calculated the
+same on both sides so is calculated as
+        Pstream::nProcs()*max(myProcNo, neighbProcNo)
+      + min(myProcNo, neighbProcNo)
+which is
+- unique
+- commutative
+- does not interfere with the default tag (= 1)
 
 
 - when constructing a GeometricField from a dictionary it will explicitly