TECHNICAL_DOC // BLOCKCHAIN / HARD-FORK
HARD
FORK
FORK
A fork/">hard fork is a backward-incompatible change to Bitcoin's consensus rules
that permanently diverges the blockchain if not universally adopted. Old nodes reject
blocks valid under the new rules, creating two separate chains and two separate coins.
Every hard fork requires unanimous adoption to avoid a permanent split
— making contentious hard forks effectively chain splits.
HARD_VS_SOFT_FORK
RULE CHANGE COMPATIBILITY
Hard fork — LOOSENS rules (old nodes reject new blocks):
Old rule: block size ≤ 1 MB
New rule: block size ≤ 8 MB ← expands what's valid
Old node sees 4 MB block → INVALID (rejects it)
New node sees 4 MB block → VALID (accepts it)
Result: network splits immediately
Soft fork — TIGHTENS rules (old nodes accept new blocks):
Old rule: any signature script valid
New rule: specific signature format required
Old node sees new-format block → still VALID (accepts)
New node enforces additional check
Result: single chain if majority hashrate upgrades
Hard forks REQUIRE all nodes to upgrade to maintain
one chain. Soft forks only require miner majority.
Notable Bitcoin Hard Forks
HISTORY
Several contentious hard forks have created permanent splits from Bitcoin, resulting in separate coins with shared transaction history up to the fork block.
Bitcoin Cash (BCH) — August 1, 2017 (block 478,558):
Change: block size limit from 1 MB to 8 MB (later 32 MB)
Result: permanent chain split, BCH as separate coin
Holders received 1 BCH per 1 BTC at split height
Bitcoin SV (BSV) — November 15, 2018:
Split from BCH over block size and protocol direction
Further fragmented the BCH community
Bitcoin Gold (BTG) — October 24, 2017:
Changed PoW algorithm from SHA256 to Equihash
Intended to remove ASIC mining advantage
Emergency consensus hard forks (no chain split):
2010: Integer overflow bug fix (block 74,638)
2013: BDB lock limit caused accidental 0.8/0.7 split
These were fixed quickly with broad consensus
TERMINOLOGY_INDEX
Hard Fork
A backward-incompatible consensus rule change. Old nodes reject blocks valid under the new rules.
Chain Split
The result of a contentious hard fork: two separate blockchains diverging from a common history.
Replay Attack
Broadcasting a transaction valid on both chains after a split. Mitigated by replay protection (different sighash).
Rule Loosening
Hard forks expand what is considered valid — old nodes cannot validate new-rule blocks.