Your chart of accounts, journals for everything the cycles do not post automatically, and an audit trail behind every figure. This is the part that makes every other figure trustworthy.
Most small‑business books cannot survive a question. There is no structure, no entry to trace, and last month changes every time somebody opens the file.
Every transaction in every subsystem posts to accounts you defined, with a trail back to its source.
It's the part your accountant judges you on. Here's how M‑Accounting works with their chart of accounts and their way of closing.
Start free, load your own chart of accounts and opening balances, and close a real period before you pay anything.