A block is a structured record that groups transactions with metadata and links to an earlier block. Blocks are proposed under a network’s protocol rules and checked independently by participating nodes; inclusion does not make every detail of a block permanently irreversible at once.
What a block contains
A block commonly has a header and transaction data. Depending on the protocol, the header can commit to a parent block, transaction summaries, a timestamp, and consensus-specific information. Bitcoin uses a Merkle root to summarize transactions; Ethereum blocks also carry execution and consensus data. The exact format is chain-specific.
How blocks are proposed and checked
In proof-of-work systems, miners search for a valid proof under the network’s target. In proof-of-stake systems, a selected proposer creates a block and validators attest under the protocol. Full nodes check the block and its transactions against consensus rules. Producing a candidate and validating it are related tasks, but they are not the same role.
Block size, weight, and capacity
Networks set different limits on the data or computation a block may contain. Bitcoin’s SegWit-era rules use block weight rather than a simple universal megabyte ceiling; Ethereum constrains execution through a gas limit. A larger capacity can allow more activity in a block, while also affecting bandwidth, storage, verification, fees, and decentralization trade-offs. A single transactions-per-second figure does not describe every workload.
Forks and reorganizations
Two valid blocks can compete for the same position, and nodes may temporarily see different tips. Consensus resolves which branch is accepted; a reorganization can replace recent blocks. Earlier history becomes harder to alter as additional consensus-backed history accumulates, but block data should not be described as physically impossible to change.
Social Media Sentiment
- A 2017 Bitcoin forum proposal to raise the block-size limit at a chosen height drew discussion about coordination, node policy, miner support, and reorganization risk. It is a historical debate around one proposal, not a measure of today’s Bitcoin community. the 2017 forum debate over a proposed block-size change
Last updated: 2026-10
Related Terms
See Also
- What Is MEV?
- Are Layer 2 Networks Actually Decentralized?
- What Happens to Bitcoin When All 21 Million Are Mined?
- ChangeNOW
Sources
- Bitcoin Developer Guide: Block Chain — Describes Bitcoin block data, Merkle roots, previous-header hashes, and independent node validation.
- Ethereum.org: Blocks — Explains Ethereum block contents, execution, and why its production model differs from proof-of-work mining.