tl;dr: A handbook documenting patterns for building money-handling software, organized around three principles: no invented data, no lost data, and no trust. It covers money representation (precision, rounding, currencies, FX), ledger design (double-entry, event sourcing, immutability, audit trails), executing flows (idempotency, reservations, resumability), integrating with external systems (webhooks, outbox/CDC, reconciliation), access controls (segregation of duties, four-eyes), and testing strategies (property-based, crash injection, golden tests). Includes a glossary of fintech terminology for newcomers, and is presented as a living document open to contributions.