Track 1 · Mechanics · lesson 1

Blocks and the chain

7 min

You know the shape now: each block commits to the one before it. This lesson opens a block up and names the parts, because every one of them comes back later.

Each block stores the hash of the one before it. That is the chain.

A block is a header plus a list

The list is the transactions. The header is small — a few hundred bytes — and it is what actually gets hashed and chained.

The header carries:

parent hash — the link backwards
height — how many blocks came before it
timestamp — roughly when it was produced
state root — a fingerprint of the entire world after this block
transactions root — a fingerprint of the list below

Notice that the header does not contain the transactions, only a fingerprint of them. That separation is doing real work: you can verify a block's place in the chain by checking a few hundred bytes, without ever downloading the transactions themselves.

Phones rely on this. So do bridges between chains.

Predict

The header includes a state root — a fingerprint of every account balance after this block. Why store that, when the transactions alone would let you recompute it?

So you can check the result without replaying all of historyBecause transactions are deleted after a whileTo keep balances secret

Height is not time

Blocks are numbered, and that number — the height — is the closest thing a chain has to a clock. Ethereum aims for a block every 12 seconds; Bitcoin every ten minutes.

The timestamp in the header is self-reported by whoever produced the block, and only loosely policed. Contracts that need a reliable clock cannot lean on it too hard, which is a trap you will exploit in the Breach Lab.

Check

Which part of a block is hashed to produce the hash that the next block points at?

The full block, header and every transactionOnly the headerOnly the list of transactions

Worth remembering

A block is a small header plus a list of transactions; only the header is hashed and chained.
The header holds the parent hash, height, timestamp, state root and transactions root.
The state root fingerprints the world after the block, so you can check a balance without replaying history.
Height is the chain's clock. The timestamp is self-reported and only loosely policed.