> 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-user-guides/mrcm-maintenance-tool-user-guide/faqs/the-expression-constraint-language-ecl.md).

# What is the Expression Constraint Language (ECL)

The *Expression Constraint Language* is a formal syntax for representing SNOMED CT expression constraints.

Expression constraints are computable rules used to define a bounded sets of clinical meanings represented by either precoordinated or postcoordinated expressions.

Expression constraints can be used to restrict the valid values for a data element in an EHR, as the intensional definition of a concept-based reference set, as a machine processable query that identifies a set of matching expressions, or as a constraint that restricts the range of an attribute defined in the SNOMED CT concept model.

The [Expression Constraint Language - Specification and Guide](broken://spaces/sOJBD7YbxAy9bD1Ko9L9/pages/V7TAAramwXseIdqJEzP6) has full details on ECL, including use cases, requirements, logical model, syntax specification, examples, and implementation considerations.


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