TERM_DEF // TRANSACTION / MARKER
MARKER
Marker. The 0x00 byte that signals a SegWit-serialized transaction (followed by the flag byte).
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.
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_MARKER_IS
Marker — at a glance
TRANSACTION
Marker is part of the SERIALIZATION sub-family of transaction-level concepts. The byte 0x00 right after the version field that signals a SegWit transaction. The 0x00 byte that signals a SegWit-serialized transaction (followed by the flag byte).
Why it exists
DESIGN
Old nodes need a way to recognise a SegWit tx without modifying their parser. The marker byte sits where the input count would be (always non-zero for legacy txs) — old nodes interpret marker=0 as "0 inputs", which is invalid, so they ignore the tx. New nodes recognise the special marker and parse the witness section.
HOW_IT_WORKS
Mechanism
HOW IT WORKS
Marker is always the literal byte 0x00 immediately after the 4-byte version. If absent, the tx is parsed as legacy. If present, the next byte (flag) must be 0x01 (currently the only valid flag value).
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
Marker — SERIALIZATION
EXAMPLE
Legacy tx: 02000000 ...
^ 4-byte version, then directly input count
SegWit tx: 02000000 00 01 00000000
^^ marker (0x00)
^^ flag (0x01)
This is exactly why BIP-144's encoding works:
- Legacy parsers expect an input count after the version → 0x00 is "0 inputs" → invalid → ignored.
- SegWit parsers see 0x00 0x01 and switch to the new format.
KEY_PROPERTIES
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
IRREVERSIBLE
After ~6 confirmations (~1 hour), reversing the tx would require more proof-of-work than the honest network produces.
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:
TERMINOLOGY_INDEX
TERMINOLOGY
Marker
The 0x00 byte that signals a SegWit-serialized transaction (followed by the flag byte).
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)
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.