BIP Draft: Terminology of Transaction Components by murchandamus · Pull Request #2195 · bitcoin/bips

murchandamus · 2026-06-09T01:00:34Z

This informational BIP provides a set of terminology to refer to the various components, scripts, and concepts used to compose Bitcoin transactions.

This proposal was discussed in 2023 on the mailing list and had subsequently been reviewed and developed against my personal BIPs repository. I recently picked it up again to address the outstanding review and would now like to put it forth for broader consideration.

Note on pull request organization: During this initial review phase, I will append commits for each change to make it easier for reviewers to check whether their comments have been addressed adequately. The PR will be reorganized into one or a few commits as this document approaches publication.

Todos:

Add definition for the Witness Reverved Value (BIP141) (Sjors)
Inconsistency of Condition Script (see BIP Draft: Terminology of Transaction Components #2195 (comment), vostrnad)
Inconsistency of use of Forwarding Script (see BIP Draft: Terminology of Transaction Components #2195 (comment)), vostrnad)

vostrnad · 2026-06-09T22:24:10Z

+: Synonym: <code>VarInt</code> (do not use, see [[compactsize|Rationale]])
+
+; condition script
+: [Concept] Umbrella term for scripts that encode the actual spending conditions that must be fulfilled to spend a TXO. Condition scripts may be preceded by one or more forwarding scripts. An output script will either directly contain a condition script in the case of a bare output, or commit to one or multiple condition scripts for other output types. For P2TR both the taproot internal key and each leaf script in the script tree are considered condition scripts. At the time of writing, there are five distinct condition scripts: taproot output key, leaf script, witness script, redeem script (only non-segwit P2SH), and bare output script (P2PK, P2PKH, P2MS, and arbitrary scripts).


I think this is somewhat contradictory in multiple ways:

If a "condition script" is indeed meant to be a script (and the first sentence explicitly confirms it) then the taproot internal key and the taproot output key aren't condition scripts – the former is a part of the control block (itself a witness stack item that's not a script) and the latter is a witness program (a data push in an output script, not a script itself).

It says a taproot internal key is considered a condition script, but then doesn't include it in the (presumably exhaustive) list of condition scripts.

vostrnad · 2026-06-09T22:24:29Z

+: [Artifact] A serialization artifact indicating features used by the transaction. As of writing, the only allowed value is <code>01</code> which indicates that the transaction serialization has a witness structure (see [[https://github.com/bitcoin/bips/blob/master/bip-0144.mediawiki|BIP144]]).
+
+; forwarding script
+: [Concept] A collective term for scripts that redirect input validation to another script or data structure. Witness programs and P2SH Programs are forwarding scripts. Forwarding scripts make use of script templates that imply additional evaluation steps beyond the explicitly expressed conditions. In the case of P2SH, the output script itself only explicitly specifies that the redeem script must be the preimage of the hash in the output script, but the template prescribes that the redeem script must additionally be satisfied. For witness programs, the output script is even less verbose with more implied meaning.


Same problem as above: the witness and P2SH programs aren't scripts, they're byte arrays pushed in an output script. A SegWit/P2SH output script is the forwarding script in this case.

murchandamus

@vostrnad: Thank you for the great review. I took most of your suggestions, I’m still working on the Condition Script and Forwarding Script point.

murchandamus · 2026-06-10T22:48:34Z

+: See '''input script'''.
+
+; sequence
+: [Component] The sequence number appearing in each transaction input. Serialized as the four-byte <code>nSequence</code> integer field. If any input’s sequence number is less than the maximum allowed value, a transaction is non-final.


I realize that I did this for marker, sequence, version, locktime, and sighash-flag. It feels a bit out-of-place in a list of definitions, so I was considering to remove it, but would you think that it should also be added to other components instead?

jonatack

At first glance looks complete and ready for assignment.

danielabrozzoni

I only did a first (rather quick) pass. Thanks for taking the time to write this! It was very informational, and helped me clear many of the doubts I had regarding terminology :)

danielabrozzoni · 2026-06-12T09:32:03Z

+: Amount underscores that we are referring to a count of satoshis rather than subjective value. As Bitcoin transactions (and code altogether) are littered with “values”, we prefer “amount” as being less ambiguous.
+
+; <span id="compactsize"></span><code>CompactSize</code> vs <code>VarInt</code>
+: The Bitcoin Core codebase uses the term <code>CompactSize</code> for a “variable-length integer encoding used in Bitcoin Core’s serialization framework” that has outside of Bitcoin Core also been referred to as <code>VarInt</code><ref>Also see Bitcoin Stack Exchange: [[What is the different between CompactSize and VarInt encoding?|https://bitcoin.stackexchange.com/a/114585/5406]]. Since the Bitcoin Core codebase uses another type of variable-length integer encoding called <code>VarInt</code> in the context of database interaction, we prefer using <code>CompactSize</code> in the context of transaction serialization. This approach is also preferred by some technical writers working on widely read technical resources concerned with Bitcoin.


In the Bitcoin Stack Exchange url: in mediawiki format, the URL should go first, and the displayed text as second; here they're backwards:

Should be:

[[https://bitcoin.stackexchange.com/a/114585/5406|What is the different between CompactSize and VarInt encoding?]]

danielabrozzoni · 2026-06-12T09:55:49Z

+: Synonym: coin
+
+; version
+: [Component] Part of the transaction header that specifies the transaction version. Serializes as the four-byte <code>nVersion</code> integer field.


What if here we added links to the various BIPs specifying tx versions? BIP68 for version 2 and BIP431 for version 3

danielabrozzoni · 2026-06-12T10:10:49Z

+: Synonym: inputs, <code>vin</code>, <code>txins</code>, <code>tx_ins</code>
+
+; transaction output (TXO)
+: [Component] A transaction component that consists of an amount and an output script. Adds a new unspent transaction output (UTXO) to the UTXO set that is uniquely<ref>Well… usually uniquely. There have been two transactions that were repeated which created duplicate UTXOs (see BIP30).</ref> identified by an outpoint.


Should link to BIP30:

duplicate UTXOs (see [[https://github.com/bitcoin/bips/blob/master/bip-0030.mediawiki|BIP30]])

murchandamus added the New BIP label Jun 9, 2026

murchandamus mentioned this pull request Jun 9, 2026

BIP draft: Terminology for transaction components and aspects murchandamus/bips#1

Closed

15 tasks

Add transaction terminology bip

5491adc

murchandamus force-pushed the 2022-04-tx-terms branch from bbb3268 to 5491adc Compare June 9, 2026 01:19

Sjors reviewed Jun 9, 2026

View reviewed changes

Comment thread bip-tx-terminology.mediawiki

murchandamus commented Jun 9, 2026

View reviewed changes

Comment thread bip-tx-terminology.mediawiki

vostrnad reviewed Jun 9, 2026

View reviewed changes

murchandamus added 3 commits June 10, 2026 12:45

Fix compactsize link

be997fe

Split keypath and scriptpath to two words

a5d8bbb

Address vostrnad Review

1f9b42b

murchandamus commented Jun 10, 2026

View reviewed changes

Use shorter lines outside of lists

99a59aa

jonatack added the Needs number assignment label Jun 11, 2026

jonatack reviewed Jun 11, 2026

View reviewed changes

Comment thread bip-tx-terminology.mediawiki Outdated

jonatack reviewed Jun 11, 2026

View reviewed changes

Comment thread bip-tx-terminology.mediawiki Outdated

murchandamus added 2 commits June 11, 2026 10:33

Fix grammar error in article

4c5d730

Amend UTXO synonyms

03b513c

danielabrozzoni reviewed Jun 12, 2026

View reviewed changes

Conversation

murchandamus commented Jun 9, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

vostrnad Jun 9, 2026

Choose a reason for hiding this comment

Uh oh!

Uh oh!

vostrnad Jun 9, 2026

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

murchandamus left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

murchandamus Jun 10, 2026

Choose a reason for hiding this comment

Uh oh!

jonatack left a comment • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

danielabrozzoni left a comment

Choose a reason for hiding this comment

Uh oh!

danielabrozzoni Jun 12, 2026

Choose a reason for hiding this comment

Uh oh!

danielabrozzoni Jun 12, 2026

Choose a reason for hiding this comment

Uh oh!

danielabrozzoni Jun 12, 2026

Choose a reason for hiding this comment

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

5 participants

murchandamus commented Jun 9, 2026 •

edited

Loading

jonatack left a comment •

edited

Loading