What Is a Hard Fork and Soft Fork? Bitcoin Cash, Ethereum Classic, and Why Blockchains Split

TL;DR: A blockchain fork occurs when the rules governing a blockchain change. A soft fork is a backward-compatible upgrade where old nodes can still participate in the updated network without upgrading immediately. A hard fork is a non-backward-compatible change that creates a permanent split, producing two separate blockchains that share history up to the fork point but diverge afterward. The most significant hard forks in crypto history are the 2016 Ethereum split that created Ethereum Classic (ETH vs ETC), and the 2017 Bitcoin split that created Bitcoin Cash (BTC vs BCH). Hard forks are significant for holders because they typically receive coins on both chains after a split, meaning a holder of 1 Bitcoin before the Bitcoin Cash fork received 1 BTC and 1 BCH after the fork. MediaCrypto note: understanding forks is essential for understanding crypto governance, because forks are the primary mechanism through which disagreements about a blockchain's direction get resolved when consensus cannot be reached.

To understand what a fork is, it helps to understand what governs a blockchain in the first place. There is no central authority that decides Bitcoin's rules. Instead, the network runs on software that every node (computer participating in the network) must run. The rules in that software determine what transactions are valid, how blocks are structured, and what the network considers legitimate.

When participants disagree about what the rules should be, the disagreement must be resolved somehow. In conventional organizations, a CEO or board makes the decision. In a decentralized blockchain, the resolution mechanism is a fork.

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What Is a Soft Fork

A soft fork is a change to blockchain rules that is backward-compatible. Old nodes that have not upgraded can still participate in the network and validate transactions under the new rules, because the new rules are a subset of the old rules rather than a replacement.

The most significant soft fork in Bitcoin's history was SegWit (Segregated Witness), activated in August 2017. SegWit changed how transaction data is organized to increase capacity without changing the block size limit directly. Nodes that had not upgraded could still function on the network. The change was backward-compatible because old nodes simply saw the new transaction format as valid even without understanding the change fully.

Soft forks are the preferred upgrade path when possible because they do not require unanimous adoption and do not create a chain split. The network upgrades gradually as more nodes update their software.

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What Is a Hard Fork

A hard fork is a change to blockchain rules that is not backward-compatible. Old nodes that have not upgraded will reject blocks produced under the new rules, and new nodes will reject blocks produced under the old rules. If a significant portion of the network follows each set of rules, two separate chains persist simultaneously, each with its own transaction history from the fork point forward.

Both chains share the same history up to the block where the fork occurred. After that block, they are separate blockchains that evolve independently. The chain that the majority of miners or validators follow typically inherits the original name and ticker symbol, while the minority chain adopts a new name.

Hard forks happen for two primary reasons. The first is planned upgrades where the change is significant enough that backward compatibility is impossible or impractical, and the developer community has reached consensus to upgrade. Ethereum's Merge in September 2022 was a planned hard fork that switched the network from Proof of Work to Proof of Stake. The second is contentious disagreements where a portion of the community wants to go in a different direction and the disagreement is irresolvable, leading to a permanent chain split.

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The Ethereum Classic Split: The DAO Hack Fork of 2016

The most philosophically significant fork in crypto history followed the DAO hack of June 2016. The DAO was a decentralized venture fund on Ethereum that raised $150 million in ETH. A hacker exploited a smart contract vulnerability and drained approximately $60 million in ETH into a child contract.

The Ethereum community faced an impossible choice. Rolling back the hack required a hard fork that would alter the blockchain's history, violating the principle that blockchain transactions are immutable. Allowing the hack to stand meant accepting that $60 million in ETH had been permanently stolen through a code exploit.

The majority of the Ethereum community, led by core developers including Vitalik Buterin, voted for the hard fork that reversed the transactions. The hard fork activated at block 1,920,000, returning the stolen ETH to the original DAO investors.

A minority of the community rejected the fork on principle, arguing that the blockchain should be immutable regardless of the circumstances. This minority continued running the original Ethereum chain, which became Ethereum Classic (ETC). The forked chain, where the hack was reversed, retained the Ethereum (ETH) name and the vast majority of network participants, developers, and economic activity.

Ethereum Classic and Ethereum have diverged significantly since 2016. ETH switched to Proof of Stake in 2022. ETC continues on Proof of Work. ETH has the dominant developer ecosystem, DeFi, and institutional adoption. ETC has a much smaller ecosystem and has suffered multiple 51 percent attacks due to its lower hash rate.

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The Bitcoin Cash Split: The Block Size War of 2017

Bitcoin's most significant hard fork emerged from a years-long debate about how to scale the network. By 2017, Bitcoin's block size limit of 1 megabyte was causing transaction backlogs and high fees during periods of high demand. The community split between two factions.

The small-block faction argued that keeping blocks small preserved decentralization, since large blocks would require more powerful computers to run nodes, potentially concentrating control among fewer participants. This faction favored off-chain scaling solutions like the Lightning Network.

The big-block faction argued that larger blocks were the straightforward solution to Bitcoin's scaling problem and that concerns about centralization were overblown. They wanted to increase the block size limit to allow more transactions per block at lower fees.

When consensus proved impossible, the big-block faction executed a hard fork on August 1, 2017, creating Bitcoin Cash (BCH) with an initial block size of 8 megabytes (later increased to 32 megabytes). Every holder of Bitcoin at the fork point received an equal amount of Bitcoin Cash, with both chains sharing history up to block 478,558.

Bitcoin (BTC) retained the vast majority of miners, developers, exchanges, and economic activity. Bitcoin Cash has maintained a following among those who believe it represents the original Bitcoin vision of peer-to-peer electronic cash, but its market cap has remained a small fraction of Bitcoin's.

Bitcoin Cash itself subsequently forked in November 2018, splitting into Bitcoin Cash (BCH) and Bitcoin SV (BSV), following another block size disagreement between Roger Ver (BCH) and Craig Wright (BSV). Bitcoin SV later became largely irrelevant following Craig Wright's legal difficulties and the community's rejection of his claims to be Satoshi Nakamoto.

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What Happens to Your Coins During a Hard Fork

When a hard fork creates two chains, every holder of the original coin at the moment of the fork receives an equal amount of the new coin. This is because both chains share the same transaction history up to the fork block, meaning the new chain recognizes your balance from before the fork.

If you held 1 Bitcoin before the Bitcoin Cash fork on August 1, 2017, you held 1 BTC and 1 BCH after the fork. The total dollar value of both together was not necessarily different from the pre-fork value, because the market distributed the previous market cap across two assets rather than doubling it.

Accessing your new coins requires having control of the private keys for the address holding the coins at the fork time. Exchange custody simplifies this: most major exchanges automatically credited customers with the new coin after significant forks. Self-custody requires using wallet software compatible with the new chain and sometimes requires technical steps to claim the forked coins safely.

The primary risk during fork events is replay attacks, where a valid transaction on one chain can be broadcast on the other chain and accepted there as well, potentially allowing your coins to be spent on both chains without your authorization. Exchanges and wallet providers implement replay protection to prevent this, but users handling fork coins independently should understand the risk.

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About the Author

This article was researched and written by the MediaCrypto editorial team. MediaCrypto is a cryptocurrency news and market analysis publication covering Bitcoin, Ethereum, altcoins, regulatory developments, and market trends. Follow us on X at @MediaCrypto_AI and on Instagram.

FAQ — Hard Fork and Soft Fork Explained

What is the difference between a hard fork and a soft fork? A soft fork is a backward-compatible upgrade where old nodes can still participate without upgrading. A hard fork is a non-backward-compatible change that creates a permanent chain split if a significant portion of the network follows each set of rules. Soft forks upgrade the network. Hard forks can divide it.

What caused the Ethereum Classic fork? Ethereum Classic was created in 2016 when the Ethereum community split over how to respond to the DAO hack. The majority supported a hard fork reversing the stolen transactions. A minority rejected this as a violation of blockchain immutability and continued the original chain, which became Ethereum Classic (ETC). The forked chain retained the Ethereum (ETH) name.

What caused Bitcoin Cash? Bitcoin Cash was created in August 2017 by the big-block faction in Bitcoin's scaling debate. Unable to reach consensus on increasing Bitcoin's block size limit, they executed a hard fork creating Bitcoin Cash with an 8 megabyte block size. Every Bitcoin holder at the fork received an equal amount of Bitcoin Cash.

Do I get free coins during a hard fork? Yes. When a hard fork creates two chains sharing the same history, every holder of the original coin at the fork block receives an equal amount of the new coin. Accessing those coins through an exchange is typically automatic. Accessing them in self-custody requires compatible wallet software for the new chain.

What is a replay attack in a fork? A replay attack during a fork occurs when a valid transaction on one chain is broadcast on the other chain and accepted there as well, potentially spending your coins on both chains without authorization. Exchanges and wallet providers implement replay protection mechanisms. Users handling fork coins independently should verify replay protection before transacting.

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Read also: What Is a Blockchain A Simple Explanation for Beginners — https://mediacrypto.ai/news/what-is-a-blockchain-a-simple-explanation-for-beginners

Read also: The Biggest Crypto Hacks and Collapses in History What Actually Happened — https://mediacrypto.ai/news/the-biggest-crypto-hacks-and-collapses-in-history-what-actually-happened

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