# Source of truth

> The one system whose copy of a record wins when two tools disagree. Why it matters for opt-outs and phone numbers, and how a one-way sync keeps it that way.

A source of truth, also called a single source of truth or system of record, is the one system whose copy of a record wins when two tools disagree, and where a change is made first. Every other tool gets a copy from it, so the question of which phone number is current always has one answer.

## Why it matters

When the same contact lives in a CRM, an email tool and a spreadsheet, the copies drift apart as soon as someone edits one of them. Nobody notices until it costs something: a call to an old number, or a sales email to someone who asked to stop hearing from you. Without one named source of truth, every disagreement becomes a guess, and whoever is in a hurry makes it.

## An example

An illustrative case. A small agency keeps its leads in the CRM and sends its newsletter from a separate email tool. A lead clicks unsubscribe on the newsletter, so the email tool marks her as opted out, and the CRM never hears about it. A month later she gets a sales sequence from the CRM. The same week, a rep takes her new phone number on a call and types it into the shared sheet, so the CRM still shows the old one. Three tools now hold three versions of one person, and nobody can say which is right.

## How a system handles it

When I set one up, I write down which system is the source of truth for each kind of record, and every change gets made there first. Every other tool gets a one-way copy. An unsubscribe, wherever it happens, is recorded in the source first, and the sync checks it before copying a contact anywhere, so someone who asked to stop stays stopped. A scheduled check compares the copies, and anything that drifted gets flagged for a person to decide.

*Diagram: Here the CRM is the system of record: it takes every change first, then copies it one way to the email tool and the sheet.*

## Questions

**Is the CRM always the source of truth?**

It's the usual choice when your team works in it all day. If the CRM is where the duplicates are, a separate record store can be the source and the CRM gets a copy. What matters is that everyone knows which system it is and makes changes there first.

**Why not sync both ways?**

A two-way sync lets both tools change the same record, so it needs a rule for every conflict, and conflicts are where records get lost. I start one-way because it has fewer ways to go wrong, and carry changes back only for the fields that need it, like an opt-out.

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## Related

- [Dedupe key](https://signalnine.io/glossary/dedupe-key): A dedupe key is the field a system uses to decide two records are the same person or company. Which key to pick, an example, and what goes wrong without one.
- [How do I stop website form duplicates in HubSpot?](https://signalnine.io/integrations/website-form-to-hubspot): Where the handoff from your website form to HubSpot makes duplicates or loses leads, which field is the key, what stays with a person, and what I'd pick.
- [Two tools, two truths: which system is the source of truth](https://signalnine.io/bottlenecks/source-of-truth): When two tools disagree about a client, pick one owner tool for each field, let changes flow one way from it, and send edits made elsewhere back to the owner.
- [CRM hygiene](https://signalnine.io/glossary/crm-hygiene): CRM hygiene is keeping CRM records accurate, complete and free of duplicates. Why it decays, what it breaks, and how a system keeps it as a routine.
- [Records from forms and emailed PDFs, kept in one place that stays right](https://signalnine.io/systems/data-intake-sync): A system I built that keeps the right version of each customer record in Airtable. Forms and emailed PDFs feed it, and HubSpot copies from it.

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Mashrur Rahman, Founder, SignalNine. Published 2026-10-04, updated 2026-10-05.
Canonical: https://signalnine.io/glossary/source-of-truth