What is a single source of truth?
A single source of truth is an arrangement where everyone in a business relies on the same trusted version of key data and reports, so decisions are not argued over whose spreadsheet is correct.
The phrase is sometimes used as if it means putting everything into one giant system. In practice it rarely does. It means that for each important question, like how many open jobs we have, what was billed last month or which leads came in this week, there is one agreed place and one agreed definition. Systems of record supply the data; the single source of truth is the shared view people actually use.
For a small firm, that view might be a dashboard built on the CRM and accounting system, a weekly report generated automatically, or a well-maintained shared sheet with locked formulas. What matters is that leadership meetings start from the same numbers, definitions are written down, and someone owns accuracy. Old copies should be archived so nobody reaches for them by habit.
Getting there usually involves cleanup before any new tool. Duplicate records, inconsistent stage names and side spreadsheets have to be reconciled or retired. It also involves habits: if staff keep updating a private spreadsheet because they do not trust the shared view, the problem is trust and training, not software.
Example
For example, an architecture firm's principals each tracked project fees in their own spreadsheets, and the monthly partner meeting started with reconciling them. The firm agrees on definitions for fee earned and fee billed, builds one report from its project accounting tool, and retires the personal sheets. The office manager owns monthly accuracy checks.