# What is the Block Header
As transactions occur on the **Bitcoin, ergo the BSV network**, they are collected by **miners** and the P2P network (every node in the P2P network, even the non mining ones). Miners permanently record new transactions on the blockchain in an ordered set called a **block**.
Each block has a **standard structure**: it starts with a **block header**, followed by a list of **transactions** arranged in the order chosen by the miner who created the block.
#### **The Structure of a Block Header**
The **block header** is an **80-byte string** that contains six key data fields. Each serves a specific purpose in linking the block to the rest of the chain and ensuring its integrity.
Each field contributes to **validating**, **chaining**, and **securing** the block within the broader Bitcoin ledger.
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#### **How Blocks Are Chained Together**
Each new block refers to the **HASH256 of the previous block’s header**, forming a continuous sequence — the **blockchain**.
This **linkage** ensures that:
* Every block depends on the one before it.
* A change in any earlier block alters its hash, breaking the chain.
* Therefore, tampering with historical data is **immediately detectable**.
This creates what’s known as a **Write Once, Read Many (WORM)** database — a record that can be read and verified infinitely but never rewritten or falsified.
***
#### **Role of the Merkle Root**
The **Merkle Root**, included in the block header, represents all transactions in that block.
It acts as a **digital fingerprint** for the entire dataset.
This allows a **Merkle Proof** to confirm whether a transaction is included in a block — without needing to download or inspect every transaction.
It’s this mechanism that makes **simplified payment verification (SPV)** (section 8 of the Bitcoin White Paper) possible, allowing lightweight clients to operate efficiently.
***
#### **Putting It All Together**
When combined, these elements form the serialized **block header string**, such as:
`0100000097ef723d53804e4a81a6926e52115d2163c91da0516d174515070000000000001e9034c07807ebe6e0f90ec453571bba196116d3ead22cff2d51f4855ebbebbb13118552111a09ec042921834576`
***
#### **Key Takeaway**
The **block header** serves as the compact, cryptographically secure summary of an entire block.
It links to the previous header, incorporates the Merkle Root of all transactions, and encodes mining parameters — together forming the **chain of immutable truth** that defines the **Bitcoin ledger**.
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