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Clinical expressions using SNOMED CT concepts can be of two types: precoordinated expressions, which use a single SNOMED CT concept identifier; and postcoordinated expressions, which contain more than one SNOMED CT identifier.

SNOMED CT support of the postcoordination technique allows additional clinical detail to be represented if required. For example, |pneumococcal pneumonia| has a |finding site| of |lung structure|, which can be refined to |right upper lobe of lung|.

Postcoordination greatly increases the depth of detail that SNOMED CT can represent without having to include every possible specific site for every possible disorder via a concept. For example, the concept |bacterial pneumonia| has a defining relationship specifying its |causative agent| as |bacteria| and this can be refined to |Streptococcus pneumoniae|.

SNOMED CT expressions are a structured combination of one or more concept identifiers used to represent a clinical idea in a logical manner, which is automatically processable. Expressions are represented using the SNOMED CT compositional grammar, which is a lightweight syntax for the representation of SNOMED CT expressions.

The logic on which the SNOMED CT concept model is based allows alternative representations of the same or similar information to be recognized and compared. For example, |pneumococcal pneumonia| refined by |finding site||right upper lobe of lung| can be computed to have the same meaning as |right upper lobe pneumonia| refined by |causative agent||Streptococcus pneumoniae|.

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