Standardness

Standardness is the set of relay-policy checks a transaction must pass for a Bitcoin Core node to accept it into the mempool and forward it to peers. These rules are deliberately stricter than the consensus rules that govern whether a transaction is valid in a mined block. A transaction can be perfectly valid by consensus yet "non-standard," meaning ordinary nodes will neither relay it nor mine it from their own mempool — though a miner who receives it out of band may still include it.

What the Rules Check

Standardness covers many conditions: recognized script templates (P2PKH, P2WPKH, P2TR, bounded bare multisig), a maximum transaction weight, the dust limit on outputs, the sigops ceiling, a single OP_RETURN data output historically capped in size, and minimum-fee thresholds. Transactions using unknown scriptPubKey forms or non-canonical encodings are rejected as non-standard.

Why Policy and Consensus Differ

Keeping policy stricter than consensus lets node operators defend against denial-of-service and UTXO-bloat vectors without requiring a soft fork or risking a chain split. Because policy is a local node setting, operators can loosen or tighten it; this is also how new transaction types are trialled in policy before any consensus change. Pinning attacks and fee-bumping behavior both flow from these rules.

Standardness is the gatekeeper of the mempool. See Mempool Policy for the broader framework and Ancestor and Descendant Limits for how chains of unconfirmed transactions are bounded.