Forks History

Bitcoin has undergone several protocol upgrades through both soft forks and hard forks. This document provides a table of all major forks in Bitcoin's history.

Understanding Forks

The fundamental difference between soft forks and hard forks comes down to one thing: are the consensus rules being tightened or loosened?

	Soft Fork	Hard Fork
Consensus rules	Tightened (more restrictive)	Loosened (more permissive)
Old nodes	Accept new blocks (still valid under old rules)	Reject new blocks (invalid under old rules)
Backward compatible	Yes	No
Chain split risk	Possible if contentious	Guaranteed without full upgrade

Soft Fork

A soft fork tightens the consensus rules. Blocks valid under new rules are always valid under old rules, so old nodes continue to accept them.

Example: SegWit made certain transaction formats invalid that were previously valid. Old nodes still accept SegWit blocks because they don't violate the old (looser) rules.

Can soft forks cause chain splits? Yes, in certain scenarios:

UASF (User Activated Soft Fork): If nodes enforce rules that miners don't follow, the chain can split. The 2017 BIP 148 UASF threatened a split if miners didn't signal for SegWit.
Miner non-compliance: If significant mining power produces blocks violating new soft fork rules, upgraded nodes reject those blocks, potentially causing a split.
Contentious activation: When there's community disagreement about whether to activate.

Hard Fork

A hard fork loosens the consensus rules (or changes them incompatibly). Blocks valid under new rules may be invalid under old rules, so old nodes reject them.

Example: Bitcoin Cash increased the block size limit from 1 MB to 8 MB. Old nodes reject these larger blocks as invalid, guaranteeing a chain split.

Complete Fork History Table

Date	Block Height	Type	Name	BIP(s)	Description	Status
2009-01-03	0	-	Genesis Block	-	Bitcoin network launch	Active
2010-08-15	74,638	Hard Fork	Value Overflow Incident	-	Fixed integer overflow bug	Resolved
2012-04-01	173,805	Soft Fork	P2SH (Pay-to-Script-Hash)	BIP 16	Enabled complex scripts via script hashes	Active
2013-03-11	225,430	Accidental	BerkeleyDB Fork	-	Database lock limit caused chain split	Resolved
2013-03-12	225,430	Soft Fork	Strict DER Encoding	BIP 66	Required strict DER signature encoding	Active
2013-05-15	250,000	Soft Fork	Strict Multisig	BIP 65	Required NULLDUMMY in multisig	Active
2015-12-08	388,381	Soft Fork	CLTV (CheckLockTimeVerify)	BIP 65	Enabled absolute time locks	Active
2016-07-04	419,328	Soft Fork	CSV (CheckSequenceVerify)	BIP 112, BIP 68, BIP 113	Enabled relative time locks	Active
2017-08-01	478,558	Hard Fork	Bitcoin Cash	-	Increased block size to 8 MB	Split Chain
2017-08-24	481,824	Soft Fork	Segregated Witness (SegWit)	BIP 141, BIP 143, BIP 144, BIP 148	Separated witness data, fixed malleability	Active
2018-11-15	556,766	Hard Fork	Bitcoin SV	-	Bitcoin Cash fork, increased to 128 MB blocks	Split Chain
2021-11-14	709,632	Soft Fork	Taproot	BIP 341, BIP 342, BIP 340	Schnorr signatures, improved privacy	Active

Activation Mechanisms

Soft Fork Activation Methods

BIP 9 (Version Bits)
- Used for: CSV, SegWit, Taproot
- Requires: 95% of blocks signal support
- Grace period: 2016 blocks (~2 weeks)
- Example: Taproot activated via BIP 9
IsSuperMajority (Legacy)
- Used for: P2SH, CLTV
- Requires: 75% of last 1000 blocks
- Example: P2SH activated via IsSuperMajority
User-Activated Soft Fork (UASF)
- Used for: SegWit (BIP 148)
- Community-driven activation
- Example: SegWit UASF movement

Hard Fork Activation

Hard forks typically require:

All nodes to upgrade simultaneously
Or acceptance of chain split
Coordination among miners and nodes

Fork Statistics

Soft Fork Adoption

Fork	Activation Date	Current Usage	Adoption Rate
P2SH	2012-04-01	Multisig, complex scripts	~15-20% of transactions
CLTV	2015-12-08	Time locks, escrow	~1-2% of transactions
CSV	2016-07-04	Lightning Network	Critical for LN
SegWit	2017-08-24	Most transactions	~80% of transactions
Taproot	2021-11-14	Modern wallets	~5-10% of transactions

Hard Fork Results

Fork	Original Chain	New Chain	Current Status
Bitcoin Cash	Bitcoin (BTC)	Bitcoin Cash (BCH)	Both chains active
Bitcoin SV	Bitcoin Cash (BCH)	Bitcoin SV (BSV)	Both chains active

Timeline Visualization

2009 ──────────────────────────────────────────────────────────────
     │ Genesis Block
     │
2010 ──────────────────────────────────────────────────────────────
     │ Value Overflow Fix (Hard Fork)
     │
2012 ──────────────────────────────────────────────────────────────
     │ P2SH (Soft Fork)
     │
2013 ──────────────────────────────────────────────────────────────
     │ BerkeleyDB Fork (Accidental)
     │ Strict DER, Strict Multisig (Soft Forks)
     │
2015 ──────────────────────────────────────────────────────────────
     │ CLTV (Soft Fork)
     │
2016 ──────────────────────────────────────────────────────────────
     │ CSV (Soft Fork)
     │
2017 ──────────────────────────────────────────────────────────────
     │ Bitcoin Cash (Hard Fork) ──► BCH
     │ SegWit (Soft Fork)
     │
2018 ──────────────────────────────────────────────────────────────
     │ Bitcoin SV (Hard Fork) ──► BSV
     │
2021 ──────────────────────────────────────────────────────────────
     │ Taproot (Soft Fork)
     │
2024+ ──────────────────────────────────────────────────────────────
     │ Future upgrades...

Key Takeaways

Soft Forks are Preferred: All major protocol upgrades since 2012 have been soft forks
Backward Compatibility: Soft forks maintain backward compatibility with old nodes
Chain Splits: Hard forks always create splits; soft forks can also split the chain if contentious (e.g., via UASF)
Activation Methods: Different activation mechanisms (BIP 9, IsSuperMajority, UASF) with varying risks
Gradual Adoption: New features take time to reach full adoption

Potential Future Soft Forks

Covenants: Restrict how coins can be spent
OP_CTV (CheckTemplateVerify): Transaction templates
SIGHASH_ANYPREVOUT: More flexible signature types
Ephemeral Anchors: Lightning Network improvements

For more details on script types and their evolution, see Script System.