Block

Anatomy of a block

A block has two main parts:

• Header — a small data structure containing the block's metadata: previous block's hash, Merkle root of all transactions in the block, timestamp, mining difficulty target, and a nonce that miners adjust to find a valid hash.
• Body — the list of transactions included in the block.

The previous-block hash is what makes the chain a chain. Each block points back to the one before it; modifying any earlier block invalidates the hashes of every subsequent block, requiring all of them to be re-mined.

Block size and frequency

Different blockchains tune the trade-off between throughput and decentralization differently:

• Bitcoin — ~10 minute blocks, currently around 1.5–2 MB after Taproot/SegWit accounting, ~7 transactions per second.
• Ethereum — 12-second slots, variable size targeted at 30 million gas units (~150 transactions in a typical block), ~15 transactions per second on L1.
• Solana — 400ms slots, much larger throughput.
• Sui, Aptos, Monad — sub-second finality, parallel execution targeting tens of thousands of transactions per second.

The trade-off: bigger and faster blocks let networks process more activity but require more bandwidth and storage to validate, raising the cost of running a full node and pushing toward fewer validators.

How blocks get added

In proof-of-work chains, the next block is added by whichever miner finds a hash below the difficulty target first. They broadcast the block; other nodes verify and accept it. Miners are economically incentivized to extend the longest chain rather than fork.

In proof-of-stake chains, validators take turns proposing blocks based on their stake. Other validators attest to the proposal's validity; once enough attestations accumulate, the block finalizes.

Reorganizations

In rare cases, two miners produce a valid block at the same height, causing a temporary fork. The network resolves it by extending one branch faster than the other; the orphaned block's transactions go back to the mempool. Bitcoin sees occasional 1-block reorgs and very rare deeper ones. PoS chains with explicit finality (Ethereum after the Merge) reach a state where reorgs beyond a certain depth are economically impossible without massive validator slashing.

Confirmations

When a transaction is included in a block, it has "1 confirmation." Each subsequent block adds another confirmation. More confirmations = more buried in chain history = harder to reverse via a 51% attack or reorg. Bitcoin exchanges typically require 3–6 confirmations for deposits; high-value transfers wait for more.