 What is the difference between a fork of the Bitcoin Core reference client and a fork of Bitcoin? That's an excellent question. In fact, we use the word fork to describe three or four different scenarios. First of all, creating a fork in software, which is a very common thing. You see it on GitHub and other software projects. It means simply creating an alternative version of that project, where different developers are pursuing different goals. That doesn't necessarily change the consensus rules, the behavior on the network, or anything like that. There are in fact a number of Bitcoin Core reference client forks that are entirely compatible with Bitcoin Core and operate on the same Bitcoin network, and do not cause a fork of Bitcoin because they use the same consensus rules. One example of that is the Knotz Distribution, which is a fork of Bitcoin Core by Luke Dasher. That fork simply has some different software distribution policies, but it follows the same consensus rules as Bitcoin Core. You will see Bitcoin Knotz clients operating on the Bitcoin network without any problem, and interoperating with Bitcoin Core. That's a software fork. Another thing you might see is a change that causes a network fork, where the fork in the network splits in two. You can have that even without a change in the consensus rules. It can cause quite a few weird problems. Finally, the one you are most likely thinking of is a consensus fork. A consensus fork is a change in the consensus rules. There are two types of consensus forks. A consensus fork, which is a soft fork, where the rules have become more restrictive, and therefore clients following the old rules can still follow the new rules. A hard fork, where the rules have become less restrictive, which means the clients following the old rules will find the new rules to be invalid in certain circumstances. That's the difference between a soft and a hard fork. Those are consensus forks, which means the consensus rules have changed either in a backwards-compatible or non-backwards-compatible way. Why is a new currency created when forks happen? Why can't we keep using the Bitcoin currency on the new protocol, even if it's a new fork? The reason for that, is when a hard fork happens, the rules have changed. When the rules have changed, the new rules are not compatible with the old rules. That's why it's called a hard fork, which means you can't simply move currency from one network to the other. The two networks become distinct. They have forked, and they have forked hard. They are now separate blockchains with separate rules. Therefore, as soon as you have separate blockchains with separate rules, and the currencies are no longer interchangeable, you can't keep using the same currency, because inevitably the prices are going to diverge. They behave differently, and they are going to be priced differently. The rules are different. You can't transfer one to the other without doing an exchange, which makes it so that they are, in effect, different currencies. It's not that there are technical barriers to prevent you from treating it as the same currency, it's that they are economic and technical barriers. You have a divergence of the consensus rules, a divergence of the networks, a splitting of the liquidity, a splitting of the fungibility, and a splitting of the economics. Was Bitcoin unlimited a separate blockchain because the nodes accepting bigger block sizes would have been banned by the nodes operating on Bitcoin Core? Yes and no. First of all, Bitcoin unlimited was a change in the consensus rules, and the nodes accepting bigger block sizes are running different consensus rules. These consensus rules are less restrictive than before. Before the change in Bitcoin unlimited, a one megabyte block was acceptable. After the change, a greater than one megabyte block was acceptable, which is a lesser restriction. When you have a lesser restriction, that means that nodes that are expecting a one megabyte block, if they see something bigger than one megabyte block, see that as invalid. That means that's a hard fork consensus rule change. Not only would Bitcoin Core nodes refuse to accept the blocks produced on that network as valid, but this would also, as a secondary effect, cause a network fork, where the nodes that are propagating these larger blocks will get banned. As far as the Bitcoin Core nodes are concerned, these nodes are propagating invalid blocks. The change in consensus rules, the hard fork in the consensus rules, then causes a split in the network fork, because those nodes get banned for propagating invalid blocks. Does the Bitcoin network ever go down for maintenance or system upgrade? No, it doesn't. Unlike the banking system, Bitcoin is up 24 hours a day, 7 days a week, 365 days a year. The current uptime for the Bitcoin network is 99.99% and higher. In fact, you can check, there's a nice website that can tell you that. I'm going to check the URL to make sure it's correct. There we go. Bitcoinuptime.com, currently functional for 99.992145168% of the time since the inception, January 3rd, 2009. Where is that missing 1,000th of a percentage point? It's actually probably a disruptive fork that happened in April of 2013, due to a bug in the system that delayed things for about 25 blocks. Again, the system didn't go down, it just slowed down, and then all of the transactions eventually went through, but they were just delayed by 25 blocks. How does the community come to consensus about changes in the rules, particularly soft forks? If people vote with their CPU power, does this mean that only miners who really have a say? What about developers who don't mine, or a silo mining? How do they represent their view? At first glance, it appears that miners are the only ones who have a say, but that quickly falls apart once you realize that miners need to pay their electricity bills. In order to pay their electricity bills, they need to be able to transact in the currency that they're mining. In order to transact in that currency that they're mining, they need exchanges, wallets, and merchants. In order for those exchanges, wallets and merchants to work, there have to be some users who are willing to buy the currency that these miners are now selling. In order for those users to be buying that currency, they have to be following the same rules. If miners implemented a change, the rest of the economy in Bitcoin rejected, miners would continue to mine, and they would produce a blockchain with these new rules. But they wouldn't be able to sell the currency on that blockchain, because none of the exchanges would accept it. None of the wallets would use it, and no one would buy it, because everybody is still operating by a different set of rules. That means that those who are operating by a different set of rules, exchanges, wallets, users, buyers, merchants, etc., no longer have miners mining their chain, which would definitely slow that chain down, perhaps even stop it for a while. But if that economy still has economic purchasing value, some of the miners might decide that they might want to mine that chain, where the rewards have really gone up, because there isn't that much competition for mining. Those miners would actually make better profits, so they would defect and start working for the economic majority. As soon as that happens, you immediately see how things balance out pretty quickly. Consensus is a tricky thing to understand, but one of the most important aspects of it is that there are multiple constituencies of consensus. Developers who write the rules but can't force anyone to run them. Miners who mine blocks but can't force anyone to buy the currency or accept those blocks as valid. Wallets that can produce transactions but can't force any miners to mine them, or any users to use the wallets. Merchants who sell products but can't force anyone to buy those products or give them the currency they want, etc. Everybody seems to have power, but they only have power if they play by the majority rules. If they stop playing by the majority rules, they are the ones who are losing out economically. As a result, consensus is something where you can have a lot of power as long as you keep doing what the majority says. The moment you try to go against the majority, that power becomes very weak, very quickly, and comes with significant economic costs. That's how consensus stays together.