What is a block?

Think of a ledger in which every new page is linked to the one before it: that’s roughly how a blockchain’s history is organized.

Each block contains data, usually a batch of transactions, and a header with the information the particular network requires. Among it is the cryptographic hash of the previous block, something like its digital fingerprint. The exception is the first block in the chain, the so-called genesis block, which has no predecessor.

If the contents of an old block change, its fingerprint changes too. As a result, the link to the next block no longer matches. Linking blocks by hash makes tampering detectable, while the network’s consensus rules determine which history is accepted.

Who adds the next “page”? It depends on the network. In Bitcoin, miners compete through Proof of Work. On Proof of Stake networks, validators take on the job of proposing new blocks. Full nodes independently check whether blocks follow the rules. A block doesn’t become valid just because someone created it.

Why it matters

When you send crypto, broadcasting the transaction to the network doesn’t mean it has already been recorded on the chain. It usually waits to be included in a block first. In Bitcoin, that inclusion counts as the first confirmation; each subsequent block built on top of it adds another.

How often blocks are produced and how many transactions fit in them affects waiting times and competition for the available space. We find it useful to distinguish sending from confirmation: tapping “send” in your wallet doesn’t mean the transaction has been recorded.

What to watch out for

Not all networks have the same block size, block time or way of reaching finality. Also, a recent block can in some cases be replaced because of a chain reorganization. That’s why an exchange may wait for more confirmations before crediting a deposit. In a block explorer you can check which block your transaction went into and its status.