Output (Transaction Output)

A transaction output specifies an amount of BTC and a locking script (technically the scriptPubKey) that defines what's required to spend it later. Once the transaction confirms, the output becomes a UTXO in the global unspent set, sitting on the chain until someone produces a transaction input that satisfies its script.

On the wire, each output is two fields: an 8-byte value in satoshis, followed by a length-prefixed locking script. Outputs are ordered within a transaction; the position of each output (the vout index, starting at 0) becomes part of its permanent identity. Future spends reference an output as the pair (txid, vout).

The locking script can be simple ("the holder of this private key can spend") or arbitrarily complex (multisig, time-locked, hash-locked, or general Bitcoin Script logic). A handful of standard patterns dominate practice:

• P2PKH - legacy addresses starting with 1. Spend by revealing a public key plus a matching signature.
• P2SH - addresses starting with 3. Spend by revealing a redeem script (which itself defines the real spending conditions) and its satisfying data.
• P2WPKH - SegWit addresses starting with bc1q.... Native-SegWit equivalent of P2PKH.
• P2WSH - SegWit addresses starting with bc1q... (longer). Native-SegWit equivalent of P2SH.
• Taproot (P2TR) - addresses starting with bc1p.... Adds key-path and script-path spending with Schnorr signatures.

Outputs are indivisible. You can't spend half a UTXO; you spend the whole thing and direct the leftover back to yourself in a new change output. This is why most transactions you make have two outputs: one to the actual recipient, one back to your own wallet.

Two special cases worth knowing:

• Dust outputs. Outputs below a relay-fee-dependent threshold (commonly ~546 sats for legacy types, smaller for SegWit and Taproot) are non-standard - the default mempool policy refuses to relay them. Wallets won't create them.
• OP_RETURN outputs. A locking script starting with OP_RETURN is provably unspendable. They carry zero satoshis and exist only to embed small data commitments - protocol metadata, attestations, sidechain anchors - without bloating the UTXO set.