What is a block?
A block is a data package that a blockchain uses to group new records. It usually contains transactions or other updates, along with information that nodes use to check them. Except for the first block, a new block also usually carries a digital fingerprint of the previous block, called a hash. These references connect the blocks in order.
What a block contains
Blocks commonly contain three kinds of information:
- Transactions or other updates: the new records that the block adds to the chain.
- A link to the previous block: usually the previous block's hash, which helps nodes check the order of the blocks and detect changes to earlier history.
- Information needed to check the block: for example, summaries of transactions or state, timing or ordering information and data required by the network's consensus mechanism.
This is a general pattern, not a universal format. Fields, capacity limits and validation rules vary between networks.
Bitcoin example: According to the Bitcoin Developer Reference, a Bitcoin block consists of an 80-byte header and a transaction list. The header contains a version, the previous block header hash, a Merkle root, a timestamp, a difficulty target and a nonce. The Merkle root acts as a summary fingerprint of the block's transactions: changing committed transaction data changes that fingerprint. For blocks containing SegWit transactions, BIP-141 defines a separate commitment to witness data. It is recorded in the coinbase transaction and linked to the block header through the transaction Merkle root.
The first transaction in a Bitcoin block is the coinbase transaction, which claims the permitted block subsidy and transaction fees. An “empty block” may contain no ordinary user transactions, but it still contains a coinbase transaction. The Bitcoin Developer Guide describes this structure.
Ethereum example: An Ethereum proof-of-stake block contains consensus-layer information and an execution payload. The execution payload records transactions and the results needed to check their execution. The Ethereum blocks documentation explains that the block gas limit caps total gas use rather than setting a fixed number of transactions.
Merkle roots, state roots and parent hashes are digital fingerprints used to check data; they are not copies of all the underlying data. A node still needs the relevant block contents or state information to perform a full check.
How a block enters network history
A new block does not become part of the network's accepted history merely because it has been announced. It usually passes through these stages:
- A miner or validator gathers transactions into a candidate block.
- The candidate is sent to other nodes.
- Nodes check whether the block and its transactions follow the protocol rules.
- If two valid blocks appear at about the same time, nodes use the network's rules to decide which block to build on.
- As later blocks or validator votes arrive, that history usually becomes harder to change. Some networks also mark it as finalized.
These network rules are decision methods defined in the protocol. Bitcoin nodes select a valid chain by accumulated proof of work, while Ethereum uses validator attestations. Other networks may use different methods. A block can therefore pass validation without becoming part of the history the network ultimately follows.
Position and identification
Block height describes a block's position in a blockchain, much like a page number. Two competing blocks can have the same height, so height alone does not uniquely identify a block.
A block hash is a digital fingerprint calculated from block data and is used to identify a particular block. Some block explorers call height a “block number,” but that number does not by itself show whether a block is accepted or finalized. The first block in a blockchain is called the genesis block and has no previous block.
Transaction status inside blocks
A transaction is included when it appears in a block. If the network continues to add blocks after that block, the transaction's confirmation count usually increases. A transaction reaches finality when the network's rules treat that part of history as not changing under normal conditions.
A network can sometimes switch to a different recent block history. This is called a chain reorganization. Transactions in replaced blocks may wait for inclusion again, appear in another block or become invalid. Ethereum also establishes finality through validator votes on checkpoints. “Included,” “confirmed” and “finalized” therefore describe different stages.
Block capacity
A block does not hold a fixed number of transactions. Transactions differ in size and in the operations they require, just as the same-sized box can hold different numbers of differently sized objects.
Bitcoin measures capacity using block weight as defined in BIP-141. Ethereum uses a block gas limit to cap total gas use. Two blocks near their respective limits can therefore contain different numbers of transactions.
Block time
Block timing depends on the network. In proof-of-work systems, finding a block is probabilistic, so the interval between blocks is not identical each time. Ethereum divides time into fixed slots, but a slot can be empty if its selected validator does not successfully propose a block. The Ethereum blocks documentation explains this process in more detail.
Block validation
Nodes check a block for themselves rather than accepting it simply because it has been broadcast. They check matters such as whether it links to a valid previous block, whether its format and transactions follow the rules and whether it stays within capacity and other limits. The exact checks depend on the network.
If a check fails, a node ignores the block and continues processing other valid blocks. Wide propagation cannot turn an invalid block into a valid one.
FAQ
Is a block the same as a blockchain?
No. A block is one data package. A blockchain is the record formed when blocks are connected according to the network's rules.
Is a block the same as a block reward?
No. A block stores transactions and related information. A block reward is compensation that some networks provide to block producers under their protocol rules.
Is an “empty block” completely empty?
Not necessarily. In Bitcoin, an “empty block” usually means a block with no ordinary user transactions, but it still contains a coinbase transaction.
Related concepts
Read about blockchains, blockchain addresses and gas fees, or return to the Crypto Glossary.
Sources
- Bitcoin Developer Reference: block chain — block header fields, transaction commitments and serialized structure.
- BIP-141: Segregated Witness — witness data, witness commitments and block weight.
- Bitcoin Developer Guide: block chain — validation, coinbase transactions, chain selection and competing blocks.
- Ethereum: blocks — block fields, execution payloads, gas limits, slots and timing.
- Ethereum: proof-of-stake — proposals, attestations, fork choice and finality.
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