> ## Documentation Index
> Fetch the complete documentation index at: https://docs.narrative.io/llms.txt
> Use this file to discover all available pages before exploring further.

# ID Mapping

> Connecting identifiers across devices, channels, and platforms

ID mapping connects different identifiers to the same user, enabling cross-device targeting, measurement, and analytics. When a data provider links a hashed email to a mobile advertising ID, or a cookie to a device identifier, they create an ID mapping that bridges previously siloed data.

## Why ID mapping matters

Modern consumers interact with brands across multiple touchpoints:

* Mobile apps (identified by MAID)
* Websites on desktop (identified by cookies)
* Email communications (identified by email address)
* Connected TV (identified by device graphs)

Without ID mapping, each touchpoint exists in isolation. A retailer might show the same ad to a customer on mobile, desktop, and CTV without recognizing it's the same person—wasting budget and creating a poor experience.

ID mapping solves this by creating links between identifiers, enabling:

* **Unified targeting:** Reach the same person across devices
* **Frequency capping:** Limit ad exposure across channels
* **Attribution:** Connect conversions to the right touchpoints
* **Audience expansion:** Find customers in new channels

***

## Common ID linkages

### Hashed Email to MAID

The most valuable and deterministic mapping connects hashed emails to mobile advertising IDs.

**How it works:** When a user logs into a mobile app with their email address, the app developer can link the MAID (from the device) to the hashed email (from the login). This creates a deterministic, high-confidence link.

**Use cases:**

* Target email subscribers in mobile apps
* Extend CRM audiences to mobile advertising
* Measure mobile conversions against email campaigns

### Cookie to MAID

Connects web browsing behavior to mobile device identity.

**How it works:**

* Cookie syncing through pixel fires on mobile web
* SDK-based matching when users interact on both web and app
* Probabilistic matching based on shared signals (IP, location, behavior)

**Use cases:**

* Retarget website visitors in mobile apps
* Build unified user profiles across web and app
* Measure cross-platform campaign performance

### Cookie to Hashed Email

Links browser-based identity to email-based identity.

**How it works:** When users log in on websites, the site can link their cookie to their hashed email. This creates a durable identifier that persists even when cookies are cleared.

**Use cases:**

* Build persistent identity across browser sessions
* Connect anonymous browsing to known customers
* Enable email-based audience activation

***

## ID mapping quality

Not all ID mappings are equal. Consider these quality factors:

### Deterministic vs. probabilistic

| Type              | Description                                               | Confidence |
| ----------------- | --------------------------------------------------------- | ---------- |
| **Deterministic** | Direct observation of link (e.g., same user logged in)    | High       |
| **Probabilistic** | Inferred from signals (e.g., shared IP, similar behavior) | Variable   |

Deterministic mappings are more accurate but harder to scale. Probabilistic mappings offer broader reach but may include false matches.

### Recency

ID mappings can become stale:

* Users reset their MAIDs
* Cookies expire or are cleared
* Email addresses change

More recent mappings are generally more reliable.

### Source quality

The method of collection matters:

* **First-party login data:** Highest quality
* **SDK integrations:** High quality
* **Cookie syncing:** Medium quality
* **Probabilistic inference:** Variable quality

***

## ID mapping in Narrative

Narrative enables ID mapping through several mechanisms:

### Standardized identifiers

When ingesting data, Narrative normalizes identifier formats to enable matching across sources:

* Consistent casing for MAIDs
* Standardized hashing for emails and phones
* Narrative Cookie IDs for cross-supplier cookie matching

### Cross-supplier matching

Different data suppliers use different cookie namespaces. Narrative creates a standardized cookie ID that links to each supplier's cookies, enabling matching across suppliers.

### Query-time joining

Use NQL to join datasets on shared identifiers:

```sql theme={null}
SELECT
  a.hashed_email,
  b.maid
FROM company_a."email_data" a
JOIN company_b."maid_data" b
  ON a.narrative_id = b.narrative_id
```

***

## Privacy considerations

ID mapping must respect privacy regulations and user expectations:

* **Consent:** Ensure mappings are created with appropriate user consent
* **Pseudonymization:** Always hash PII before creating mappings
* **Data minimization:** Only create mappings needed for specific use cases
* **User controls:** Honor opt-outs across all linked identifiers

***

## Related content

<CardGroup cols={2}>
  <Card title="Identifier Types" icon="tags" href="/concepts/identifiers/id-types">
    Categories of identifiers used in mapping
  </Card>

  <Card title="Data Onboarding" icon="upload" href="/concepts/identifiers/data-onboarding">
    Connect offline data to online identifiers
  </Card>

  <Card title="UID2" icon="shield-check" href="/concepts/identifiers/uid2">
    Privacy-preserving universal identifier
  </Card>

  <Card title="Narrative ID" icon="fingerprint" href="/concepts/security/narrative-id">
    Secure cross-partner matching
  </Card>
</CardGroup>