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Two or more concepts may be combined to form a new concept  by joining them with the "+" symbol. This is particularly useful where both concepts are primitive. The resultant expression is the child of each of the concepts in the expression . The resultant expression  below represents a drug dose form that is both a spray and a suspension.

Although not stipulated by the syntax, the two concepts joined in this way must (in nearly every case) come from the same top level hierarchy . A rare exception to this rule is those expressions which refer to products that are both drugs and devices, and are therefore descendants of both 373873005 |pharmaceutical / biologic product| and 260787004 |physical object|.

The syntax does not mandate which concepts in the expression should have associated terms and which should not, so it is valid (although not advisable) to mix and match. For example, the following syntax is valid:

   421720008 +  7946007 |Drug suspension|
The syntax allows spaces, tabs and carriage returns in most places. The following example has an identical meaning to the one above:
7946007 |Drug suspension|