Most of the blockchains we've been talking about so far have been public blockchains. Bitcoin and Ethereum are open public networks that anyone can join. All networks aren't like this, blockchains come in many varieties. When it comes to who is allowed to access them, there are three types. Public, which we've heard about, consortium or shared permission and private. Technically, both consortium and private blockchains are known as permission since they require permission to access them. So, where public blockchains allow anyone to download the software and create a node, consortium blockchains only allow certain members to be nodes. That is, nodes are granted permission to join, which is why another name for consortium blockchain is shared permission. For example, you might have a consortium blockchain in which three companies and the government regulator operate nodes. You could set up your blockchain so that all four entities have to sign transactions and all would be able to access and audit this ledger without needing access to the internal ledgers of the constituent organizations. This could be considered a low trust environment. Organizations have some trust in each other but not complete trust. Power isn't centralized with any one company but the public can't see the transactions. Private blockchains take this one step further. In a private blockchain, there is generally a high degree of trust. This is much more like a centralized system but provides a high degree of auditability since transactions are viewable but only to those with access. In many ways, private blockchains are essentially just databases. However, blockchains have built-in benefits of cryptographic auditability, more immutability and identity as it relates to transactions. Any transactions and changes can be traced over time and to specific parties. This is not necessarily the case with databases, in which data can be added, removed and modified and is not necessarily traceable. Private blockchains are of particular use to developers who can set up and control their own blockchain instance to test their software and experiment with prototypes. Note that the performance of private and consortium blockchains are often much better because they don't rely on proof-of-work to establish consensus, their environment is more trusted. Consortium blockchains are sometimes called shared permission blockchains and private called permission blockchains. That's just something to note. The key thing to know is that one is completely open and public. One restricts to a few possibly competing entities and the last, to a few or one trusted entity. Public, consortium and private. Now that you know the three main infrastructures of blockchain, we can talk about blockchain interoperability. Interoperability refers to the idea of blockchains that can interact with one another. Throughout the course so far, we've discussed each different blockchain as an ecosystem unto itself separated from all the others. There are, however, efforts to interweave blockchains with one another to provide services that work more transparently from one to the other. You could, for example, have an application on the Ethereum blockchain that works with Bitcoins and does so transparently for a user. The only thing to bear in mind here is that blockchains need not be Islands disconnected from one another, there are many projects underway that tie different blockchains together in novel and powerful ways.