2015 → 2020Bitcoin forks: the splits that tested Bitcoin

The block size war

By 2015 Bitcoin blocks were filling up. Every block was capped at one megabyte, so when demand rose, fees rose and payments waited. Two answers took shape. One camp wanted bigger blocks, so the chain itself could carry more payments. The other wanted to keep blocks small, so that anyone could afford to run a full node and check the rules, and to scale with improvements on top.

The big-block side tried to replace the software most of the network ran. Bitcoin XT, led by Gavin Andresen and Mike Hearn, came in 2015; Bitcoin Classic and Bitcoin Unlimited followed in 2016. None won enough support.

In May 2017, 58 companies and miners signed the New York Agreement: activate Segregated Witness first, then double the block size limit three months later. At the same time, users pushed a user-activated soft fork (BIP 148) that would enforce SegWit from August 1, 2017. In July miners locked in BIP 91, which made SegWit signalling mandatory. The war was about to end, and to split.

Limit at the time

1 MB per block

Main attempts

Bitcoin XT (2015), Classic and Unlimited (2016)

Compromise

New York Agreement, May 23, 2017

Bitcoin in 2017

Bitcoin Cash: the first chain that stayed

The big-block camp chose to leave. Amaury Séchet presented a plan at a conference in Arnhem on June 30, 2017 and released the Bitcoin ABC software in July. The mining pool ViaBTC announced it would mine the new coin and gave it a name: Bitcoin Cash.

On August 1, 2017, after the last shared block was mined at 13:16 UTC, the two chains went their separate ways. The first Bitcoin Cash block arrived about five hours later, from ViaBTC: 1.9 MB, larger than Bitcoin allows, with the message “Welcome to the world, Shuya Yang!”. The new rules allowed blocks up to 8 MB, and mandatory replay protection, added six days before launch, kept a payment on one chain from being copied to the other.

Because the history was shared, everyone who held bitcoin at that moment now also held the same amount of Bitcoin Cash. It first traded at roughly a tenth of Bitcoin’s price. Its early difficulty rule pushed miners to jump back and forth between the chains, which made blocks arrive in bursts, until a new adjustment algorithm took over on November 13, 2017. In May 2018 the limit rose again, to 32 MB.

Date

August 1, 2017

Block size limit

8 MB at launch, 32 MB from May 2018

Holders received

1 BCH for every BTC

The fork as an event

Meanwhile, SegWit: a soft fork that kept one chain

Three weeks after Bitcoin Cash left, Segregated Witness went live on Bitcoin at block 481,824. It changed how signatures are stored, fixed transaction malleability and made room for more transactions without raising the block size limit.

SegWit was a soft fork: it tightened the rules instead of loosening them, so older nodes still accepted the new blocks and the chain did not split. That is the key difference between a soft fork and a hard fork, and the reason Bitcoin itself has never split in two.

Activated

Block 481,824

Kind

Soft fork, no new coin

SegWit activates

Bitcoin Gold: mining for everyone

Bitcoin Gold had a different complaint. By 2017 bitcoin was mined by specialised machines (ASICs) in large farms. Bitcoin Gold, led by Jack Liao of the Hong Kong mining company LightningASIC, split off at block 491,407 on October 24, 2017 and switched to Equihash, a mining algorithm meant for ordinary graphics cards.

The launch was rough. The team kept about 100,000 coins for itself as an endowment, which drew criticism from the start, and the project’s website was hit by a denial-of-service attack.

A small chain is cheap to attack. In May 2018 someone controlled most of Bitcoin Gold’s mining power for a while, rewrote recent blocks and double-spent about 388,000 BTG, roughly $18 million, against exchanges. Bittrex asked the team to cover part of its loss; the team refused, and Bittrex delisted the coin in September 2018. Bitcoin Gold changed its algorithm again in July 2018, was attacked again in January 2020 (7,167 BTG, about $72,000), and shipped an emergency update in July 2020.

Split at

Block 491,407

Mining

Equihash, for graphics cards

Largest attack

May 2018, about $18 million double-spent

SegWit2x: the fork that never came

The New York Agreement still had a second half: a hard fork to 2 MB blocks, planned for block 494,784 in mid November. More than 80% of miners had signalled support, but the plan never won consensus among users, node operators and Bitcoin Core developers.

On November 8, 2017, Mike Belshe announced on the project’s mailing list that the fork was suspended, in a message co-signed by Wences Casares, Jihan Wu, Jeff Garzik, Peter Smith and Erik Voorhees. Some nodes kept running the software anyway. Because of an off-by-one error they split one block early, at 494,782, then waited for a block larger than 1 MB that never came. That chain stalled for good.

Planned for

Block 494,784

Signed by

58 companies and miners

Result

Cancelled, no coin

Bitcoin SV and the hash war

Bitcoin Cash then split itself. Its November 2018 upgrade divided the community: Bitcoin ABC kept a 32 MB limit and added new features, while a group led by Craig Wright and the billionaire Calvin Ayre wanted to go back to what they called Satoshi’s vision, with blocks of up to 128 MB.

The chains split at block 556,766 on November 15, 2018, and both sides pointed hashrate at each other in what became known as the hash war. Major exchanges paused Bitcoin Cash trading until it was clear which chain was which. The ABC chain kept the Bitcoin Cash name; the other became Bitcoin SV (BSV). Bitcoin Cash, about $425 the day before, traded near $289 after the split, and BSV near $96.

In April 2019 Binance delisted BSV after a public feud over Craig Wright’s claim to be Satoshi Nakamoto, and ShapeShift followed. BSV suffered 51% attacks in June, July and August 2021. In March 2024 the High Court in London ruled that Craig Wright is not Satoshi Nakamoto.

Split at

Block 556,766

Limits

32 MB (BCH) vs 128 MB (BSV)

Holders received

1 BSV for every BCH

Bitcoin in 2018

A third split: eCash

Two years later Bitcoin Cash split again. Bitcoin ABC proposed that 8% of every block reward should go to fund development, an address controlled by the ABC team. Most miners and other node teams, led by Bitcoin Cash Node, rejected it.

The upgrade on November 15, 2020 separated the two. Most miners stayed with Bitcoin Cash Node, which kept the Bitcoin Cash name; the ABC chain traded as BCHA and, in July 2021, renamed itself eCash.

Date

November 15, 2020

Cause

An 8% block reward for development

Today

Known as eCash (XEC)