A hard fork (or hardfork) is a permanent divergence in the blockchain, which occurs when non-upgraded nodes can’t validate blocks created by upgraded nodes that follow newer consensus rules. Cryptocurrencies after hard fork share a transaction history up to a certain time and date.

See also bitcoin wiki and this article to get more information about differences between soft forks and hard forks

The first intentional hard fork splitting bitcoin happened on 1 August 2017, resulting in the creation of Bitcoin Cash. Other coin split created altcoins such as Bitcoin Gold or Bitcoin Private. Owner of bitcoins automatically gets the newly created coin via coin split so if you owned bitcoins during bitcoin hard fork coin split, you also own the new coin (eg. Bitcoin Cash). If the owner wants to claim his, it has to be implemented in the wallet.

In November 2018, a hard-fork chain split of Bitcoin Cash occurred. This hard-fork resulted in the creation of Bitcoin ABC and Bitcoin SV.

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