The Bitcoin blockchain is the primary innovation in Bitcoin that makes it practical. Blockchains have applications in many contexts other than cryptocurrencies. This note is an introduction to blockchains that requires no prior knowledge, including of Bitcoin. Blockchains are ledgers of transactions kept by a set of participants, none of which is accorded special status as the “correct one.” Instead, agreement is reached by a process of consensus. I show how this works for Bitcoin, discuss applications in many alternative settings and provide some detail about a very different proof-of-concept application of blockchains by the Japan Exchange Group.