> For the complete documentation index, see [llms.txt](https://hub.bsvblockchain.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://hub.bsvblockchain.org/wiki/transactions-and-utxos/bitcoin-transactions/txid.md). # TXID A Transaction ID or TXID is the **double SHA256 hash** or **SHA256d** of a serialised Bitcoin transaction. TXIDs are not part of the transaction, as the hash cannot be generated until the transaction is complete. A TXID and VOUT (or **prevout\_n**) index are used to reference UTXOs when they are added to a transaction as an input. 1. Data ``` 1. Explanation The double SHA256 hash enables network participants to identify and communicate transactions efficiently. They are used at all levels of the ecosystem, including wallets and block explorers. Transaction IDs are used extensively in the peer-to-peer protocol. For example, peers on the network synchronise their transaction database using a 3 message sequence. A node will send out an **inv** message containing one or more transaction IDs. In the case that the receiving node/peer does not have a copy of the full transaction, he can respond with a **gettransaction** message containing the transaction ID. This is responded to with a **tx** message containing the full serialised transaction. Using transaction IDs to broadcast transaction database information, greatly reduces the volume of data that has to be sent over the network. The double hash property can be used in another interesting way. The user can know that a corresponding party has a copy of the full TX by requesting the intermediate hash, which can only be generated by hashing the full transaction. This 'proof of possession' is present across Bitcoin and enables complex functionality and checking of resources when Bitcoin is being used in a fully peer to peer environment. 1. See Also - Bitcoin Transactions - Transaction Malleability - Transaction Pools ``` --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation by asking a question. Perform an HTTP GET request on the following URL with the `ask` and `goal` query parameters: ``` GET https://hub.bsvblockchain.org/wiki/transactions-and-utxos/bitcoin-transactions/txid.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is what the user is ultimately trying to achieve, the reason they need the answer. Sharing it helps GitBook give you a better, more relevant answer. A goal is most helpful when it describes the outcome the user wants rather than restating the question. For example, with `ask=how do I create an API token`, a goal like `automate deployments from our CI pipeline` lets GitBook tailor the answer to that use case. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.