# definition of custom fields in archetypes

**Category:** [Technical (archive)](https://discourse.openehr.org/c/technical-archive/156)
**Created:** 2019-10-09 07:30 UTC
**Views:** 1
**Replies:** 1
**URL:** https://discourse.openehr.org/t/definition-of-custom-fields-in-archetypes/14429

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## Post #1 by @system

Hallo,
I have a question about the degree of freedom in defining archetypes:
Archetypes are not composed but rather specified by constraining the base classes from the reference model\. Hence no new/custom field members can be definied in the definition of a new archetype but only the field members already existing in the base classes have to be used\. So there will never be a path like

a/myField/myProperty/value

but always just something like

a/data/events/items/value

The same applies for the definition of templates\.
Is this correct ?
Greetings
Georg

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## Post #2 by @system

Hi Georg,

Yes that is absolutely correct. The archetypes act as a constraint on the RM so must always be compliant with the RM, and whatever underlying archetype class is selected. Same applies to templates. Practically speaking this does still give us a huge amount of flexibility, especially as we can add cluster archetypes in Slots, which is very useful when trying to align with legacy system data.

Ian

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**Canonical:** https://discourse.openehr.org/t/definition-of-custom-fields-in-archetypes/14429
**Original content:** https://discourse.openehr.org/t/definition-of-custom-fields-in-archetypes/14429