What is a block in a blockchain, and how are blocks linked?

Waiting in line

You send a transaction from a wallet app. The app passes it to one or more nodes, which share it with the rest of the network.¹ Sharing it does not put it on the blockchain yet.¹ On many blockchains it waits in a queue of pending transactions until it is added to a block.¹ On Ethereum, each node's list of pending transactions is called its mempool.³

What is in a block

A block has two parts: a header and the block data.¹ The block data is a list of checked, genuine transactions.¹ The header holds information about the block. Every blockchain picks its own fields, but many use these:¹

• The block number, also called the block height.¹
• The hash of the previous block's header.¹
• A single hash that stands for all of the block's data.¹
• A timestamp.¹
• The block's size and, on mining blockchains, a number called the nonce.¹

Bitcoin builds that single hash with a merkle tree. Each transaction is hashed. The hashes are paired and hashed again, and so on, until one hash is left. That last hash is the merkle root, and it goes in the header.⁴

Other nodes check every transaction in a new block. They refuse the block if any transaction is invalid.¹

How the chain is made

Each block contains the hash of the previous block's header. That link is what makes the chain.^1,4 If someone changed an old block, its hash would change, so the next block's stored link would no longer match. Fixing that link changes the next block's hash too, and so on to the newest block.^1,2 So altered blocks are easy to spot and reject.¹

The first block is called the genesis block. It has no block before it, so its "previous hash" is usually set to all zeros.¹ In Bitcoin, a block's height is the number of blocks between it and the genesis block, which is block 0.⁴

Why batch transactions?

Ethereum groups transactions into blocks so that everyone agrees on the same history, dozens or hundreds of transactions at a time.² A new Ethereum block is due every 12 seconds.² Bitcoin aims for about one block every 10 minutes.^1,5

Tamper-evident, not untouchable

People often call blockchains immutable, meaning they can never change. NIST says that is not strictly true.¹ Blockchains are tamper evident and tamper resistant: changes are easy to spot and hard to make.¹ Lesson 6 shows how the newest blocks can sometimes still be replaced.