What is hard fork?

A rule change that makes previously invalid blocks valid, so nodes that do not upgrade reject the new chain and stop following it.

Coordinated hard forks are routine on chains that plan them, but a contested one produces two chains with a shared history, which is how Ethereum and Ethereum Classic came to exist after the DAO fork and how Bitcoin Cash split from Bitcoin. For a holder, the practical questions are whether an exchange credits the new asset, when it does so, and whether withdrawals are paused around the event.

Both chains inherit the same balances, so an address exists on each, but the two are separate assets afterwards. The mistake worth avoiding is treating a fork as a free windfall. Claiming a forked asset carelessly, especially by importing a seed phrase into unfamiliar software, has cost more people more money than the forked coins were ever worth.

How it works

The distinction from a soft fork is about compatibility direction. A soft fork tightens the rules, so old nodes still accept the new blocks. A hard fork loosens them, so a block that is valid under the new rules looks invalid to an old node, and the two sets of nodes stop agreeing on which chain is real.

Most hard forks are planned upgrades. Developers publish the change, an activation block height is set, node operators and exchanges upgrade before it, and no split occurs because nobody keeps running the old rules. Chains that ship regular upgrades this way generate no new assets at all.

A contested fork is different. A group keeps mining or validating the old rules, and both chains continue with a shared history up to the split block and separate histories after it. Exchanges then decide independently whether to list the new asset, and their announcements set the practical timeline: deposits and withdrawals for the affected asset are usually suspended before the fork block and resumed once the chains are stable and replay protection is confirmed.

Replay protection matters to holders. Without it, a transaction broadcast on one chain can be valid on the other, so a spend on one side can move the mirrored balance on the other.

Example

Illustrative walkthrough. You hold 2 units of an asset on an exchange when a contested fork activates at block 800,000. The exchange halts deposits and withdrawals a day earlier and announces it will credit the new asset at a 1 to 1 ratio for balances held at the fork block. After the split you have 2 units of the original and 2 units of the new asset, and the new one may trade at a small fraction of the original or not trade at all if no venue lists it. Your original position did not double in value. It was divided by a rule change.

Why it matters when you buy

Around a scheduled fork, exchanges pause deposits and withdrawals for that asset, which can strand a transfer you started at the wrong moment. Check the venue's own announcement before moving funds, and see the change log for listing and status changes RampAtlas has recorded. If you hold in self-custody instead, the exchange pages show which venues have credited forked assets before.

Related terms

• fork: the general term for a rule divergence
• soft fork: a backward-compatible tightening of the rules
• node: the software that enforces which rules apply
• block height: how activation points are specified
• chain reorganization: a shorter-lived divergence between nodes
• seed phrase: the secret fork claim scams target

Questions

Do I automatically get the new coin?

If you hold your own keys, the balance exists on both chains and you can access it with the right software. If you hold on an exchange, you get it only if that exchange decides to credit and support it.

Why do exchanges pause withdrawals around a fork?

Until the new chain is stable and replay protection is confirmed, a withdrawal can be duplicated or reversed. Pausing is how a venue avoids crediting a transaction that later disappears.

Does a hard fork always create a new coin?

No. Planned upgrades that everyone adopts are hard forks with no split at all, because no one continues running the old rules.