In this video, I start with a recap of what a ledger is, in order to set the groundwork for later videos on the different levels that exist in blockchain.  This will help you to understand what is going on in cryptocurrencies and NFTs, and what parts are arbitrary, and which are fixed.  (Well, ultimately all of blockchain is arbitrary, because we invented it, but some parts are now fixed, and other parts can be "played with". But more on that later in the series...)