The five-century-old system where every transaction touches at least two accounts: money doesn't just leave, it leaves one place and arrives in another. The built-in symmetry makes the books self-checking; if debits and credits don't match, something is wrong.
In Bizdivers: the ledger underneath all the automation is genuine double-entry, with journal entries, a trial balance, and a general ledger a bookkeeper can verify. See The bookkeeper's toolkit.