The comitative case expresses the partner accompanying an individual in a certain action. The instrumental case marks the tool used by the agent of a lexical verb to carry out the action in the given situation. The relator morpheme may be a case ending, adposition, etc. Languages differ in whether they express each of these two functions separately.

Types:

Com=Instr: The comitative and instrumental functions are expressed the same way.

ComNotInstr: The comitative and instrumental functions are expressed differently.[1]

MixComInstr: Multiple strategies exist to express the comitative and instrumental functions. Although these strategies are regularly used to express both, at least one variant exists that can only be used to express either the comitative or instrumental. [2]

[1] Languages with additional strategies also fall into this category; for example, the use of a particular adposition is required for one of the two cases, but ungrammatical for the other.

[2] In such cases, the grammatical morpheme thought to be specific should be examined to determine whether it can actually be considered a case morpheme (not a derivational morpheme). One method is to examine the morpheme’s distribution to see whether the morpheme occurs in the same grammatical environment as other morphemes known to be case affixes.