diff --git a/proto/profile.proto b/proto/profile.proto
index 6fd1dd17..c41ec50e 100644
--- a/proto/profile.proto
+++ b/proto/profile.proto
@@ -128,7 +128,10 @@ message Sample {
 }
 
 message Label {
-  int64 key = 1;   // Index into string table
+  // Index into string table. An annotation for a sample (e.g.
+  // "allocation_size") with an associated value.
+  // Keys with "pprof::" prefix are reserved for internal use by pprof.
+  int64 key = 1;
 
   // At most one of the following must be present
   int64 str = 2;   // Index into string table