Blocks and the blockchain
How transactions are grouped into blocks and chained together into one history.
What is the blockchain?
The blockchain is Bitcoin's shared history: blocks of transactions, each linked to the one before by its hash. Learn how it works and why it is hard to change.
What is a block?
A Bitcoin block is a batch of transactions plus an 80-byte header that links it to the chain and proves the work done. See what is inside one.
The block header
The block header has six fields in 80 bytes: version, previous hash, merkle root, time, bits and nonce. Learn what each does and why miners hash it.
The genesis block
Block 0, mined on 3 January 2009, started the Bitcoin blockchain. Learn about its newspaper headline, its unspendable 50 BTC and how to view it.
Block height
Block height is a block's position in the chain, counting from 0 at the genesis block. Learn how heights schedule halvings, timelocks and confirmations.
Merkle trees and the merkle root
A merkle tree hashes a block's transactions in pairs until one hash remains: the merkle root. Learn how it works and how merkle proofs save space.
Block size and the weight limit
Bitcoin blocks are limited to 4,000,000 weight units, about 1.5 to 2.5 MB in practice. Learn why the limit exists and how it creates the fee market.
Block timestamps
Miners set each block's timestamp within two rules: later than the median of the last 11 blocks, and at most 2 hours in the future. Learn why.
Stale blocks and chain reorganizations
When two miners find a block at once, nodes briefly disagree, then all switch to the longer chain. Learn how reorganizations work and what they mean for you.
The longest chain (most work) rule
Bitcoin nodes follow the valid chain with the most accumulated proof of work, not simply the most blocks. Learn why and how this creates agreement.
Duplicate transaction IDs and the BIP 30 rule
Explore Bitcoin’s historical duplicate coinbase transactions, the UTXO overwrite problem, and why archival records need block context.
Median time past: Bitcoin’s shared time reference
Calculate median time past with an eleven-block example and understand why Bitcoin timelocks do not promise an exact wall-clock release time.