> For the complete documentation index, see [llms.txt](https://docs.snomed.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.snomed.org/snomed-ct-practical-guides/snomed-ct-extension-guide/5-key-steps/5.3-assessing-requests/5.3.2-constraints-on-concept-requests/5.3.2.2-model-metadata-concept-hierarchies/5.3.2.2.4-namespaces.md).

# Namespaces

## Namespace Concept

| **Hierarchy**              | [370136006 \| Namespace concept\|](http://snomed.info/id/370136006)                                                                                                                             |                |                                                                    |                             |                                                                    |                             |                                                                    |                             |                                                                    |                             |                 |
| -------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------- | ------------------------------------------------------------------ | --------------------------- | ------------------------------------------------------------------ | --------------------------- | ------------------------------------------------------------------ | --------------------------- | ------------------------------------------------------------------ | --------------------------- | --------------- |
| **Purpose**                | Each concept in this subhierarchy represents an allocated SNOMED CT extension namespace.                                                                                                        |                |                                                                    |                             |                                                                    |                             |                                                                    |                             |                                                                    |                             |                 |
| **Examples**               | <p></p><ul><li><a href="http://snomed.info/id/373872000">373872000                                                                                                                              | Core Namespace | </a>  </li><li><a href="http://snomed.info/id/370137002">370137002 | Extension Namespace 1000000 | </a>  </li><li><a href="http://snomed.info/id/370138007">370138007 | Extension Namespace 1000001 | </a>  </li><li><a href="http://snomed.info/id/384597007">384597007 | Extension Namespace 1000002 | </a>  </li><li><a href="http://snomed.info/id/413335000">413335000 | Extension Namespace 1000003 | </a> </li></ul> |
| **Concept Addition Rules** | <p>No new subtype children or descendants may be added in an extension.</p><p>Namespaces concepts can only be created by SNOMED International to represent allocated namespace identifiers.</p> |                |                                                                    |                             |                                                                    |                             |                                                                    |                             |                                                                    |                             |                 |

Please see the section on [Logical Design](/snomed-ct-practical-guides/snomed-ct-extension-guide/4-logical-design.md) for a further introduction to namespaces.

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