# What is the Block Header As transactions occur on the BSV network, they are collected by miners to be permanently written to the BSV ledger in an ordered set called a block. These blocks have a standard structure which is the block header followed by the set of transactions in the order determined by the miner who created the block. ![](https://bitcoinsv.academy/storage/photos/8383/BSVA-MerkleTrees_Ch3Less1_VA1_cropped_1645188650.jpg) The block header is an 80-byte string of specific data elements. Firstly, a 4 byte version parameter followed by a 32 byte HASH256 of the previous block's header, the 32 byte Merkle root created from the Merkle tree containing all of the transactions in the block, a 4 byte timestamp indicating when the block was created, a 4 byte floating point number indicating the difficulty target for the proof of work process and a4 byte 'nonce' or 'number used once' which is used in the puzzle that miners compete to solve. ![](/files/m1ggHtxUvzrHs11dMHXU) | **FIELD** | **PURPOSE** | **BYTES** | **FORMAT** | **STRING** | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | --------------------------------------------- | --------- | ------------- | ---------------------------------------------------------------- | | Version | Version rules to follow | 4 | Little Endian | 01000000 | | Previous Block Hash | 256-bit hash of the previous block header | 32 | Little Endian | 97ef723d53804e4a81a6926e52115d2163c91da0516d17451507000000000000 | | Merkle Root | 256-bit hash of all transactions in the block | 32 | Little Endian | 1e9034c07807ebe6e0f90ec453571bba196116d3ead22cff2d51f4855e3bbebb | | Time | Current block timestsamp as UNIX epoch | 4 | Little Endian | 1311855211 | | nBits | Current difficulty target in compact format | 4 | Little Endian | 1a09ec04 | | Nonce | 32-bit number (starts at 0) | 4 | Little Endian | 2921834576 | | **RAW SERIALISED STRING** | | | | | | 0100000097ef723d53804e4a81a6926e52115d2163c91da0516d174515070000000000001e9034c07807ebe6e0f90ec453571bba196116d3ead22cff2d51f4855e3bbebb13118552111a09ec04292183457613118552111a09ec042921834576 | | | | | It is through the link back to the previous block header that blocks become chained together in a 'blockchain'. Thinking back to the outcome of what changing just a single bit of information changes in a Merkle tree, we can see that the entire contents of the blockchain all the way back to the Genesis block becomes a solid data silo, unchangeable and verifiable. This is how the ledger gets its status as a Write Once Read Many (WORM) database. Any attempt to change to any part of the structure would fail when subject to simple, hash-based data verification processes. In addition, the Merkle root acts as a digital fingerprint that links every transaction in a block to the block header, allowing for a Merkle proof to be used to verify the presence of any transaction in any block. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://hub.bsvblockchain.org/higher-learning/bsv-academy/merkle-trees/merkle-trees-and-the-block-header/what-is-the-block-header.md?ask= ``` The question should be specific, self-contained, and written in natural language. 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.