This is Lesson 1 of the “Cryptocurrency Systems Course,” which has 42 lessons in total. This course doesn’t provide code or teach you how to place trades. It does just one thing: break down, layer by layer, the logic behind how cryptocurrencies work, so that when you finish, you can judge for yourself whether something is reliable instead of just following someone else’s conclusions. So in this first lesson, we won’t get into market trends. We’ll tackle just one basic question: What exactly is a blockchain?

Let’s start with something familiar: a ledger.

Imagine a village with a hundred households, where people often lend money to one another and pay it back. In the past, the village had an accountant who recorded in his ledger who owed whom and how much. The problem was, if the accountant tore out or altered a page, nobody else would know. And a ledger, simply put, is just a series of records: who transferred how much of what to whom, and when. Banks record money; cryptocurrencies record coins. At their core, they’re the same kind of thing—a page-by-page transaction history.

What blockchain does is copy the same ledger to thousands of people, with everyone keeping an identical copy.

Let’s go back to the village example. Now it’s not just one accountant keeping the books: one hundred people in the village each have a ledger with exactly the same contents. One day, Zhang San wants to dodge a debt, so he secretly crosses out the line in his ledger that says, “Owes Li Si 100 yuan.” But when he’s done, he sees that the same line is still there in the other ninety-nine copies. As soon as everyone compares their ledgers, Zhang San’s copy won’t match. This is blockchain’s simplest and most fundamental strength: you can change your own copy, but you can’t change the ninety-nine copies everyone else has. This state—where no single institution has the final say, the rules are public, and all participants maintain the system together—is what people commonly call decentralization.

So what are “blocks” and “chains”?

The ledger gets too long to keep writing forever, one page at a time. So at regular intervals, the system bundles all the transactions that took place during that period into a page. That page is called a block. You can think of it as a page bound into the ledger, listing who transferred how many coins to whom during that period.

The key is how the next page is bound in. Each new page carries a tamper-proof marker based on the contents of the previous page—even changing a single word on the previous page completely changes that marker. The pages link together one after another, forming a chain. That’s where the name “blockchain” comes from.

This has a serious consequence: if you secretly change a number on page one hundred, the marker on page one hundred and one won’t match. To make the chain match again, you’d have to redo every page after page one hundred. Meanwhile, everyone else’s ledgers would still be unchanged. The cost of changing one page becomes the cost of changing the entire chain and convincing most people to change theirs too. That’s why it’s hard to tamper with—not because it involves some kind of magic, but because the fact that “changes will be detected” is built into its structure.

There’s one point beginners often get wrong, so it’s worth discussing on its own: many people assume decentralization means faster and cheaper, when actually the opposite is true.

Decentralization doesn’t solve the problem of speed; it solves the problem of “who guarantees that this page of the ledger hasn’t been changed?” The trade-off is that it’s slower and more expensive. It’s not hard to see why: if one hundred people in a village are keeping the same ledger, every entry has to be shared with everyone and checked by all of them. That’s bound to be slower than having one accountant quietly write it all down. And convincing those ninety-nine people to agree with you is much harder than convincing one accountant. So in this space, speed and decentralization are often competing priorities. Different projects make different trade-offs; there’s no single right answer, and neither side is inherently better.

So what you really need to remember from Lesson 1 is this: blockchain isn’t some new technology that’s faster. It’s a way of keeping records that makes changes impossible to hide—the ledger is copied to everyone, and each page links to the next, so a change in one place makes the copies stop matching everywhere.

📖 A new lesson is published every day. There are 42 lessons in total; you can find them all in the course index post pinned at the top.

⏭ Next lesson: Lesson 2 · Where Did Bitcoin Come From?

⚠️ This lesson explains only the principles and mechanisms. It does not recommend any coins and is not investment advice.

When it comes to “what exactly is blockchain,” you can now at least explain what it is and why it works this way—and that’s enough.