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TERM_DEF // TRANSACTION / SEGWIT_TRANSACTION
SEGWIT
TRANSACTION
transaction/">SegWit Transaction. A transaction using SegWit serialization (BIP141); separates witnesses from the TXID.

This page sits in the Transaction section — How money moves: inputs, outputs, fees, signatures, sighash flags, and the formats that wrap them. Read on for what it is, why it exists, how it works under the hood, and what to watch out for.
WHAT_SEGWIT_TRANSACTION_IS
SegWit Transaction — at a glance
TRANSACTION
SegWit Transaction is part of the SERIALIZATION sub-family of transaction-level concepts. A transaction using the BIP-141 witness format — signatures live in a separate witness field rather than scriptSig. A transaction using SegWit serialization (BIP141); separates witnesses from the TXID.
Why it exists
DESIGN
Pre-SegWit signatures were inside scriptSig, which is part of the data hashed into the txid. That made txids malleable (any third party could re-encode a signature). SegWit moves signatures out of the txid hash, fixing malleability and unlocking Lightning.
HOW_IT_WORKS
Mechanism
HOW IT WORKS
A SegWit tx has the legacy fields + a marker (0x00) + a flag (0x01) right after the version, followed by a witness section after the outputs. Legacy nodes that haven't upgraded see a stripped-down tx (no witness) — they still validate the inputs but don't check signatures.
1. Wallet selects UTXOs whose total value covers the spend amount + estimated fee (coin selection). 2. Wallet builds the transaction body: version, inputs (each with prev txid + vout + sequence), outputs (each with value + scriptPubKey), locktime. 3. Wallet computes the sighash for each input (which parts of the tx the signature commits to — controlled by the SIGHASH flag). 4. Wallet signs each input with the right private key. Witness/scriptSig is populated with the resulting signatures + pubkeys. 5. Tx is broadcast to peers. Mempool propagation: tens of seconds globally. 6. A miner includes it in a block. Confirmation count grows by 1 per block; after ~6 the tx is effectively final.
WORKED_EXAMPLE
SegWit Transaction — SERIALIZATION
EXAMPLE
Field Bytes Notes version 02 00 00 00 (4 bytes) marker 00 (SegWit indicator, byte 5) flag 01 (SegWit indicator, byte 6) input_count 01 (CompactSize) input output_count 01 output witness 02 <72-byte sig> <33-byte pubkey> (per input) locktime 00 00 00 00 txid = SHA256(SHA256(tx without marker/flag/witness)) ← stable wtxid = SHA256(SHA256(full tx including witness)) ← malleable but unused for chain identity
KEY_PROPERTIES
WITNESS DISCOUNT
Witness bytes count 1× toward block-weight/">block weight; non-witness bytes count 4×. This is the SegWit "block size increase".
ATOMIC
A transaction is either fully accepted into a block or fully rejected. There is no partial spend.
IMMUTABLE INPUTS
A UTXO can only ever be spent once. After that, it is permanently consumed.
NO BALANCES
Bitcoin tracks UTXOs, not balances. Your wallet computes a balance by summing the UTXOs it controls.
COMMON_PITFALLS
Things that catch people out
PITFALLS
  • address-reuse/">Address reuse degrades privacy — every reuse links more of your UTXOs together publicly. Modern wallets generate a fresh address per receive.
  • Fee estimation matters: under-pay and your tx sits in the mempool for hours; over-pay and you tip the miner more than necessary. Use a fee estimator.
  • "Change outputs" must go back to a fresh address you control. A missing output/">change output sends the difference to the miner as fee — a known footgun.
  • RBF (Replace-By-Fee) lets you re-broadcast a tx with a higher fee. Useful for stuck txs but means a 0-confirmation tx is never truly final.
RELATED_CONCEPTS
Other terms from Transaction — click any to read its page:
Transaction (Tx) Raw Transaction Transaction ID (TXID) wTXID (Witness TXID) Input Output UTXO (Unspent Transaction Output) UTXO Set
TERMINOLOGY_INDEX
TERMINOLOGY
SegWit Transaction
A transaction using SegWit serialization (BIP141); separates witnesses from the TXID.
Transaction (Tx)
A signed payload spending one or more UTXOs and creating new ones; every state change in Bitcoin is a tx.
Raw Transaction
The hex-serialized bytes of a transaction, ready to broadcast or analyze.
Transaction ID (TXID)
HASH256 of a transaction's pre-witness serialization; used to reference outputs by (txid, vout).
wTXID (Witness TXID)
HASH256 of the full transaction including witness data; commits to signatures and used in the witness commitment.
Input
A reference to a previous output being spent, plus the data (scriptSig/witness) authorizing the spend.
Output
An (amount, scriptPubKey) pair created by a transaction; spendable later by a tx whose input references it.
UTXO (Unspent Transaction Output)
An output that hasn't been spent yet; your "balance" is the sum of UTXOs you can sign for.