Ledger

A ledger records ownership and obligation: who has what, and who owes what to whom. That is all.

The first ones we know of were clay tablets in Mesopotamian temples, recording who had deposited how much grain. The Medici and their contemporaries refined double-entry bookkeeping, in which every transaction is written twice — once as a plus, once as a minus — so that the two sides must balance. A notebook that checks itself.

Your bank balance is a line in a ledger. When you pay for a coffee, no physical money travels: one ledger records minus five, another records plus five, and the two institutions settle between themselves later.

The idea matters far beyond banking. A blockchain is, at bottom, a ledger too — the difference is who is allowed to keep it and who is allowed to read it.