\cite{2018}

Before attempting to understand how blockchain ledgers work, it is worth taking a look at traditional ledgers. For centuries, banks have used ledgers to maintain databases of account transactions, and governments have used them to keep records of land ownership. There is a central authority – the bank or government office – which manages changes to the record of transactions, so they can identify who owns what, at any given time. This allows them to check whether new transactions are legitimate, that the same €5 is not spent twice and houses are not sold by people who don't own them. Since users trust the manager of the ledger to check the transactions properly, people can buy and sell from each other even if they have never met before and do not trust each other. The middleman also controls access to information on the ledger. They might decide that anyone can find out who owns a building, but only account holders can check their balance. These ledgers are centralised (there is a middleman, trusted by all users, who has total control over the system and mediates every transaction) and black-boxed (the functioning of the ledger and its data are not fully visible to its users). Digitisation has made these ledgers faster and easier to use, but they remain centralised and black-boxed (Fig. \ref{737334}).