A hard fork is a radical change to the protocol that makes previously invalid blocks/transactions valid, and therefore requires all users to upgrade. For example, if users A and B are disagreeing on whether an incoming transaction is valid, a hard fork could make the transaction valid to users A and B, but not to user C.

A hard fork is a protocol upgrade that is not backward compatible. This means every node (computer connected to the Bitcoin network using a client that performs the task of validating and relaying transactions) needs to upgrade before the new blockchain with the hard fork activates and rejects any blocks or transactions from the old blockchain. The old blockchain will continue to exist and will continue to accept transactions, although it may be incompatible with other newer Bitcoin clients.

A soft fork is a change to the Bitcoin protocol wherein only previously valid blocks/transactions are made invalid. Since old nodes will recognise the new blocks as valid, a soft fork is backward-compatible. This kind of fork requires only a majority of the miners upgrading to enforce the new rules.

Some examples of prominent cryptocurrencies that have undergone hard forks are the following: Bitcoin’s hard fork that resulted in Bitcoin Cash, Ethereum’s hard fork that resulted in Ethereum Classic. Bitcoin Cash has been hard forked since its original forking, with the creation of Bitcoin SV.
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