What does a Bitcoin block contain?

Card answer

Correct answer

Answer A: Transactions plus data linking the block to the previous one.

Quick explanation

Each block groups transactions together and contains information linking it to the previous block. This connects the blocks in order to form the blockchain.

Detailed explanation

In simplified terms, a Bitcoin block has two parts: the block header and the transactions. The first transaction is the miner's coinbase transaction. It is followed by the other valid transactions selected by the block creator.

The header contains, among other things, the version, previous block header's hash, Merkle root, timestamp, encoded target and nonce. The Merkle root is calculated from transaction identifiers in the block. The header thus binds the transaction list to a compact verification value. Altering an included transaction would change the Merkle root and therefore the block-header hash.

The reference to the previous block header links blocks into a chain. The header is also the part whose hash must meet the proof of work target. A block contains no centrally maintained list of account balances and never contains users' private keys. Full nodes check both the header and every transaction, accepting the block only if all their applicable consensus rules are met.

Transaction hashes are combined in pairs until a Merkle root is produced. It appears in the block header and enables compact proofs that a transaction belongs to a block without including every other transaction in the proof. Such a proof does not automatically replace a full node's complete consensus validation.

Example or everyday application

A block may contain thousands of transactions. Instead of putting each directly in the header, the Merkle root summarizes their identifiers in one verification value. A wallet receives the transaction hash and neighboring hashes along a Merkle path and can calculate the same root as in the block header.