diff --git a/SHAMapDiff.cpp b/SHAMapDiff.cpp
index 69c960493..00a29a8fe 100644
--- a/SHAMapDiff.cpp
+++ b/SHAMapDiff.cpp
@@ -2,6 +2,14 @@
 
 #include <stack>
 
+// This code is used to compare another node's transaction tree
+// to our own. It returns a map containing all items that are different
+// between two SHA maps. It is optimized not to descend down tree
+// branches with the same branch hash. A limit can be passed so
+// that we will abort early if a node sends a map to us that
+// makes no sense at all. (And our sync algorithm will avoid
+// synchronizing matching brances too.)
+
 class SHAMapDiffNode
 {
 	public: