diff --git a/metainfo/piece_key.go b/metainfo/piece_key.go
index b5aa492e..c476da61 100644
--- a/metainfo/piece_key.go
+++ b/metainfo/piece_key.go
@@ -1,5 +1,6 @@
 package metainfo
 
+// Uniquely identifies a piece.
 type PieceKey struct {
 	InfoHash Hash
 	Index    int