What is a blockchain? In plain words

Ten friends go on holiday and one of them keeps the list of who paid for what. That works until someone doubts the list, or the person holding the pen makes a small change nobody sees. Now picture all ten keeping their own copy, with a rule that every copy has to match.

That second version is the idea behind a blockchain. Replace the ten friends with thousands of computers, and the holiday list with a record of who owns what.

A notebook with thousands of copies

The easiest way to picture a blockchain is as a notebook. Many computers keep a copy. When something happens, it is written down as a new line, and every copy gets the same line.

Say Ana sends Ben 20 USDC. USDC is a digital dollar: a token built to stay near one dollar. Here is what happens to that payment:

1. The payment goes out to the computers as a new line: Ana to Ben, 20 USDC.
2. The computers check the line against shared rules. Does Ana really hold 20 USDC? Did the request really come from her?
3. New lines are collected onto a page, called a block.
4. The block is sealed to the one before it, and every copy gets the same block. Ana now holds 20 less and Ben holds 20 more.

That is the whole trip. No bank approved it. The rules did.

Why the seal matters

Each new page carries a kind of fingerprint of the page before it. That fingerprint is worked out from everything on the earlier page, so changing even one number gives a different fingerprint. This is what joins the pages into a row: a chain of blocks.

Now say someone edits an old page in their own copy, to hand themselves an extra 20 USDC. The fingerprint on the next page no longer fits, and their notebook no longer matches the others. So the other computers ignore it and carry on with the version they agree on. Nobody has to chase the change down. It simply does not count.

That is why no bank has to sit in the middle. Instead of trusting one company's record, you rely on many matching copies and rules that everyone follows.

What people get wrong

A blockchain is not a coin. It is the record. Coins and tokens are balances written in it.

It does not judge what it records. The computers check that a payment follows the rules. They cannot tell whether it was a good idea. Send 20 USDC to the wrong person and that line is as valid as any other, with no bank to call.

There is not just one. Many blockchains exist, and people often just call them chains. Each keeps its own notebook. Six of them appear in this series: Ethereum, Arbitrum, Base, BNB Chain, Polygon and Optimism. They are not all built the same way, and they do not all run on the same number of computers.

Open does not mean named. Anyone can read a public chain's notebook, but its lines show addresses, which are long strings of letters and numbers that work like account numbers. They do not show names.

What you can check yourself

Because the notebook is open, you do not have to take anyone's word for a payment. Websites called block explorers let you look up an address or a payment and see whether the line is really there. If someone says the money is on its way, the record can show you.

In short

• A blockchain is one shared record of who owns what, kept on many computers.
• New lines are gathered into blocks, and each block is sealed to the one before it.
• Change an old page and your copy stops matching, so the others ignore it.
• No bank is in charge. The computers follow shared rules.

Clear Cut is a series of one-minute DeFi explainers from Cleave Protocol.