diff --git a/bus-mapping/src/circuit_input_builder/transaction.rs b/bus-mapping/src/circuit_input_builder/transaction.rs
index 56f7b8cb0e..17aded1eca 100644
--- a/bus-mapping/src/circuit_input_builder/transaction.rs
+++ b/bus-mapping/src/circuit_input_builder/transaction.rs
@@ -194,7 +194,8 @@ pub struct Transaction {
     pub signature: Signature,
     /// RLP bytes
     pub rlp_bytes: Vec<u8>,
-    /// RLP bytes for signing
+    /// RLP bytes for signing, only used for PreEip155 tx without chainid
+    /// for other tx types: rlp == rlp_unsigned.
     pub rlp_unsigned_bytes: Vec<u8>,
     /// RLP bytes for signed tx
     pub rlp_signed_bytes: Vec<u8>,
diff --git a/eth-types/src/geth_types.rs b/eth-types/src/geth_types.rs
index 9c534e8589..b0fd969ac5 100644
--- a/eth-types/src/geth_types.rs
+++ b/eth-types/src/geth_types.rs
@@ -287,7 +287,8 @@ pub struct Transaction {
 
     /// RLP bytes
     pub rlp_bytes: Vec<u8>,
-    /// RLP unsigned bytes
+    /// RLP unsigned bytes, only used for PreEip155 tx without chainid
+    /// for other tx types: rlp == rlp_unsigned.
     pub rlp_unsigned_bytes: Vec<u8>,
     // TODO: add rlp_signed_bytes as well ?
     /// Transaction hash