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protocol/blockchain/block/block-header.md

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 # Block Header
 
+Block headers serve an important intermediary role in the creation and transmission of blocks.  They are a fixed-width (80-byte) representation of the entire block.  With a block header, you can:
+
+ 1. Calculate the hash of the block
+	 - The double SHA-256 hash of the block header
+ 2. Confirm the proof of work was executed correctly
+	 - See [Difficulty Adjustment Algorithm](/protocol/blockchain/proof-of-work/difficulty-adjustment-algorithm) for more details
+ 3. Determine the relative location of the block in the blockchain
+	 - Using the previous block hash contained in the header
+
+Since validation of all the transactions in the block can be expensive, the ability to perform these checks on the block before downloading and validating its transactions helps make denial-of-service attacks on the network significantly more expensive for attackers.
+
+## Block Header Format
+
+| Field | Length | Format | Description |
+|--|--|--|--|
+| version | 4 bytes | unsigned integer<sup>[(LE)](/protocol/misc/endian/little)</sup> | The block format version. |
+| previous block hash | 32 bytes | block hash<sup>[(LE)](/protocol/misc/endian/little)</sup> | The hash of the block immediately preceding this block in the blockchain. |
+| merkle root | 32 bytes | merkle root<sup>[(LE)](/protocol/misc/endian/little)</sup> | The merkle tree root of the transactions in the block. |
+| timestamp | 4 bytes | unix timestamp<sup>[(LE)](/protocol/misc/endian/little)</sup> | The epoch timestamp of the block in seconds. |
+| difficulty | 4 bytes | difficulty<sup>[(LE)](/protocol/misc/endian/little)</sup> | The difficulty that the block must achieve to be valid.  This value is determined by the timestamps of previously mined blocks. |
+| nonce | 4 bytes | bytes<sup>[(LE)](/protocol/misc/endian/little)</sup> | A random value that is repeatedly changes during block mining in order to achieve the block hash requirements. |