Topic
What is a block in blockchain?
Last reviewed:
- Quick fact 1
A block groups transactions and includes a link (hash) to the block before it.
- Quick fact 2
Changing an old block would break the links to every block after it.
Source: Bitcoin: A Peer-to-Peer Electronic Cash System (2008)
- Quick fact 3
On Ethereum, a new block is proposed about every 12 seconds.
Source: ethereum.org: Blocks
The short lesson
A blockchain stores its record in pages called blocks. Each block holds a batch of transactions, plus a short fingerprint, called a hash, of the block before it.
That link is what makes the chain. Block 101 carries the fingerprint of block 100, which carries the fingerprint of block 99, and so on back to the first block.
Why this matters: if someone tried to change a transaction in an old block, that block’s fingerprint would change. Then the next block’s link would no longer match, and the one after that, all the way to today. The network’s computers would reject the altered copy.
Different blockchains make blocks at different speeds. Bitcoin adds one about every 10 minutes on average. Ethereum proposes one about every 12 seconds.
When an app says your transaction has “3 confirmations,” it means your transaction is in a block and 2 more blocks have been added after it.
Related terms
Go deeper in Lesson 4: The shared list everyone can check
Common questions
What is inside a block?
A list of transactions and a header with data such as the time and the hash of the previous block.
What is block height?
The block’s number in the chain, counting from the first block.
Who creates blocks?
Miners on proof-of-work chains like Bitcoin, and validators on proof-of-stake chains like Ethereum.
Can a block be changed later?
Changing an old block would break every link after it, so the network rejects the change.