> 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-compositional-grammar-specification/appendices/appendix-a-concept-definition-and-expression-relationship-statements.md).

# Appendix A – Concept Definition and Expression Relationship Statements

SNOMED CT Compositional Grammar is a syntax for representing SNOMED CT expressions. SNOMED CT expressions are a structured combination of one or more concept identifiers used to express a clinical idea. When documenting a full concept definition or an expression relationship statement, which asserts the relationship between two expressions, it is often useful to use a syntax that is consistent with SNOMED CT Compositional Grammar. For these purposes, the following ABNF syntax extension may be used:

> statement = ws "(" ws subExpression ws ")" ws definitionStatus ws "(" ws subExpression ws ")" ws

where 'subExpression' and 'definitionStatus' are defined as per SNOMED CT Compositional Grammar.

For example, the following statement documents the definition of the fully defined concept [95617006 | Neonatal cyanosis|](http://snomed.info/id/95617006) .

( [95617006 |Neonatal cyanosis|](http://snomed.info/id/95617006) ) ===\
( [3415004 |Cyanosis|](http://snomed.info/id/3415004) + [363696006 |Neonatal cardiovascular disorder|](http://snomed.info/id/363696006) :\
[246454002 |Occurrence|](http://snomed.info/id/246454002) = [255407002 |Neonatal|](http://snomed.info/id/255407002) ,\
[363698007 |Finding site|](http://snomed.info/id/363698007) = [113257007 |Structure of cardiovascular system|](http://snomed.info/id/113257007) )

Figure 6 illustrates this concept definition using the SNOMED CT Diagramming Guideline.

<figure><img src="/files/Y2IvJ5vR8vsQiRP27iTr" alt=""><figcaption><p><strong>Figure 6: Concept definition diagram for Neonatal cyanosis</strong></p></figcaption></figure>

In this second example, the definition of the primitive concept [144008 | Normal peripheral vision|](http://snomed.info/id/144008) is shown:

( [144008 |Normal peripheral vision|](http://snomed.info/id/144008) ) <<< ( [301980006 |Finding of visual field|](http://snomed.info/id/301980006) :\
[363698007 |Finding site|](http://snomed.info/id/363698007) = [49549006 |Structure of visual system|](http://snomed.info/id/49549006) )

Figure 7 illustrates this concept definition using the SNOMED CT Diagramming Guideline.

<figure><img src="/files/58j66RdJ0gfpFspkNduG" alt=""><figcaption><p><strong>Figure 7: Concept definition diagram for Normal peripheral vision</strong></p></figcaption></figure>

In this last example, the equivalence of two expressions is defined (as illustrated in Figure 8):

( [49601007 |Disorder of cardiovascular system|](http://snomed.info/id/49601007) : [246454002 |Occurrence|](http://snomed.info/id/246454002) = [255407002 |Neonatal|](http://snomed.info/id/255407002) )\
\=== ( [64572001 |Disease|](http://snomed.info/id/64572001) : [246454002 |Occurrence|](http://snomed.info/id/246454002) = [255407002 |Neonatal|](http://snomed.info/id/255407002) ,\
[363698007 |Finding site|](http://snomed.info/id/363698007) = [113257007 |Structure of cardiovascular system|](http://snomed.info/id/113257007) )

<figure><img src="/files/UvY8gYLJzu3ZAJd0LYB3" alt=""><figcaption><p><strong>Figure 8: Diagram showing the equivalence of two expressions</strong></p></figcaption></figure>

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