Crypto With Block

Lessons / How Blockchain Works

Forks: When a Chain Splits

What a blockchain fork is, how soft and hard forks differ, and why free fork coins are the bait thieves use. Part of the How Blockchain Works series.

9 min read Free lesson Fact-checked Oct 2026

When a chain splits

Hi, I'm Block. Imagine waking up to find a brand-new coin in your wallet, one you never bought. In twenty seventeen, that really happened to everyone holding bitcoin. And within months, thieves were using free coins like that as bait. In this lesson: what a fork is, the two kinds, and how to stay safe when a chain splits.

What is a fork?

Quick recap of where this series has been. Thousands of nodes keep copies of the same ledger, and consensus is how they agree on one chain. A fork is what happens when that agreement breaks. Sometimes for a few minutes. Sometimes forever.

Let's start with the fork that happens by accident, and fairly regularly, without anyone doing anything wrong. Two miners on opposite sides of the world find a valid block at almost the same moment. Some nodes hear about one first, some hear about the other. For a short time, the network has two competing ends of the chain.

It sorts itself out with the very next block. Whichever side gets built on first becomes the longer chain with more work, and every node switches to it. The losing block is dropped. Transactions that were only in that block go back into the waiting line to be included again. That's one reason a payment isn't treated as final the instant it shows up.

Wait for confirmations before you call it final.

Sometimes the accident is bigger. Back in twenty thirteen, a new version of Bitcoin's software accepted a large block that older versions couldn't process. The network split into two chains: one run by upgraded nodes, one by older nodes. And each kept growing.

Developers and big mining pools coordinated quickly. Pools on the newer software switched back to the older version, so the chain without the big block won, and everyone came back together. But during the confusion, at least one large payment was spent twice. Even a bug can split a chain.

Even a bug can split a chain.

Changing the rules

Now for the forks people argue about: the ones on purpose. Blockchains are software, and software gets upgraded. When the rules change, there are two ways to do it. A soft fork and a hard fork. The difference is whether old software still gets along with the new rules.

A soft fork tightens the rules. Think of a club that adds a stricter dress code. Anyone dressed for the new code still meets the old one, so the old doorman lets them in too. Old nodes keep accepting the new blocks. As long as most of the network enforces the new rules, the chain stays in one piece.

Soft fork: old nodes still agree.

A well-known example is an upgrade to Bitcoin called SegWit, short for segregated witness, in twenty seventeen. It changed how transaction data was stored and made room for more transactions, without splitting the network. Most users never had to do anything.

Most users never had to do anything.

A hard fork is different. The new rules allow something the old rules forbid, so old software rejects the new blocks. If everyone upgrades, there's still one chain. Many networks upgrade this way on a schedule. But if part of the community refuses, the old rules and the new rules each keep their own chain.

Hard fork + disagreement = two chains.

And hard forks aren't always drama. Some networks make big changes through scheduled hard forks that almost everyone agrees on in advance. The people running nodes update their software, the new rules switch on at a set block, and life goes on with one chain. If you just hold coins, a planned upgrade like that usually asks nothing of you at all.

A planned upgrade usually asks nothing of holders.

Picture a road that splits. Both roads share every mile behind you. From the split forward, they lead to different places, and travelers on one can't drive onto the other. That shared history is the key to everything that comes after a split.

Same past. Different futures.

The most famous example is Bitcoin Cash. For years, people argued about how big Bitcoin's blocks should be. One side wanted bigger blocks, so each block could fit more transactions. The other side wanted to keep blocks small and add capacity in other ways, like the soft fork you just saw. On August first, twenty seventeen, the big-block side split off with its own rules.

A split over block size.

Because both chains shared the same history up to the split, anyone who held bitcoin at that moment also held the same amount of Bitcoin Cash. The same keys controlled both. Nobody sent you anything. Your old balance simply existed on two chains.

Nobody sent you anything. Your balance exists on both chains.

But here's the catch for anyone keeping coins on an exchange. Some large exchanges said up front they wouldn't support the new coin at first, and told customers to withdraw before the split if they wanted it. When an exchange holds your keys, the exchange decides what you get from a fork.

On an exchange, the exchange decides what you get.

The other famous split was about a hack, not block size. In twenty sixteen, a large fund built on Ethereum, called The DAO, was drained by an attacker who exploited a flaw in its code. Tens of millions of dollars' worth of ether was gone, and the community faced a hard choice.

Most of the network chose a hard fork that moved the stolen funds into a contract where investors could get their money back. But some said the chain's history should never be rewritten, even to undo a theft. Their motto was "code is law." They kept running the original chain, which became Ethereum Classic.

A fork is a people decision, not just a technical one.

Who decides?

Notice what those two stories have in common. Nobody can force a blockchain to change. Miners, the people running nodes, exchanges, wallet makers and everyday holders each choose which software to run and which chain to follow. When enough of them choose differently, you get two chains. A blockchain's rules hold because people keep choosing them.

The rules hold because people keep choosing them.

Forks and your safety

So that's what forks are. Now the part this channel exists for: what a split means for your safety. There are two dangers. Transactions that get copied from one chain to the other. And thieves who use free fork coins as bait.

Remember: after a split, your keys work on both chains. So right after the Ethereum split, a transaction you signed on one chain could be copied and broadcast on the other chain too. That's called a replay. You meant to move one coin, and you moved both.

The fix is called replay protection: a change that makes a transaction valid on only one chain. Ethereum Classic added it months after the split. Until then, people had to be very careful. After any contentious split, wait until your wallet clearly supports the new chain and replay protection before moving anything.

After a split, wait before you move anything.

The second danger is the bigger one. A fork means free coins, and free coins make people hurry. A few months after Bitcoin Cash, another fork called Bitcoin Gold launched. A website appeared offering to help people claim their free coins. All it needed was your private key or recovery phrase.

People typed in their phrases. The site saved them, and then emptied the wallets: not just the new coins, but the bitcoin and other coins too. More than three million dollars was taken. The official balance checker only ever asked for an address, never a phrase.

A balance check needs an address. Never a phrase.

Real fork news makes great cover for this, because the story is true and people are already paying attention. When a split is in the headlines, expect replies and messages offering to help you claim your coins before a deadline. The link leads to a page that wants your recovery phrase.

Here's the calm truth about any real fork. If you held your own keys at the moment of the split, your coins on the new chain aren't going anywhere. There's no deadline and no form to fill in. Your original coins stay right where they were, so there is never a reason to rush.

A real fork never has a deadline for you.

So when a chain splits, do less, not more. Don't move coins in the first days. Get updates only from the official wallet or project sources you already use, typed in yourself. And never type your recovery phrase into anything that offers you free coins. Ever.

Free coins and your phrase? Never together.

The people who lose money in forks are usually the ones who feel rushed: rushed to claim, rushed to sell, rushed to move. The ones who wait a few weeks, read the official announcements and let the dust settle lose nothing. Patience is a security setting too.

Patience is a security setting too.

Forks in four lines

Here's the whole lesson in four lines. Small forks happen naturally and fix themselves with the next block. Soft forks tighten rules and keep one chain. Hard forks can create two chains and two coins. And free fork coins are the bait. Your recovery phrase is the hook.

Free coins are the bait. Your phrase is the hook.

Sources

Education only. Not financial advice.

Education only. Not financial, investment, tax or legal advice. Block, our on-screen presenter, is AI-generated, voice included; the research and scripts are ours.