> 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-specifications/snomed-ct-release-file-specification/snomed-ct-identifiers/6.9-the-namespace-hierarchy.md).

# The Namespace Hierarchy

SNOMED CT core release files include metadata concepts that represent each of the allocated namespace-identifiers.

A namespace concept has the following characteristics:

* It is a subtype child of [370136006 |Namespace concept|](http://snomed.info/id/370136006)
* The fully specified name of the concept has the term
  * **Extension Namespace** *{nnnnnnn}***(namespace concept)**
* A synonym associated with each concept has the term
  * **Extension Namespace** *{nnnnnnn}*
* Where appropriate further synonyms may be included to identify the nature of the responsible organization.

{% hint style="info" %}
In the terms shown above *{nnnnnnn}* is the seven digit namespace identifier of the responsible organization.
{% endhint %}

{% hint style="warning" %}
[Visit our Namespace registry](https://cis.ihtsdotools.org/info)
{% endhint %}

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