diff --git a/api/api_full.go b/api/api_full.go
index 772a0d709..57ca5bcfd 100644
--- a/api/api_full.go
+++ b/api/api_full.go
@@ -591,6 +591,8 @@ type MsgGasCost struct {
 	TotalCost          abi.TokenAmount
 }
 
+// BlsMessages[x].cid = Cids[x]
+// SecpkMessages[y].cid = Cids[BlsMessages.length + y]
 type BlockMessages struct {
 	BlsMessages   []*types.Message
 	SecpkMessages []*types.SignedMessage