The functional domain of participation is devoted to the semantic roles born by participants of a situation. These may be subdivided into central and peripheral roles. Central roles include ‘actor’, ‘undergoer’ and, gradually more peripheral, ‘indirectus’. The set of peripheral roles is open and includes such roles as benefactive, instrumental, comitative, aversive and many more. The central roles tend to be inherent in the meaning of a verb, being associated with the positions that it opens for arguments. The peripheral roles tend to be signalled by case relators. These are only tendencies, though, and we will see exceptions.

Anyway, the grammaticalization of case relators starts with items coding peripheral roles. Such items are of necessity relational. Verbs and relational nouns are the primary candidates for being recruited as case relators. One apparent exception to this generalization is known: Ancient Indo-European languages, including Sanskrit, Hittite and, to a lesser extent, Ancient Greek have few if any pure adpositions. Instead, items bearing such meanings as ‘in’, ‘out’, ‘on’, ‘to’ etc. are adverbs. These are frequently univerbated with verbs to function as their preverbs ().

These adverbs evolve into prepositions, in this case katà N ‘down from N’. However, this is an exception that proves the rule: Adverbs like ‘down’ have a semantic argument position for their reference point. ‘Down’ means necessarily ‘down from some reference point’. This position remains deictically or anaphorically implicit in an adverb. The adposition differs from the adverb precisely in converting this semantic argument position into a syntactic complement position. Once the adverb is used as an adposition, the argument can be made explicit, in the form of a nominal complement. Greek katà as a preposition governs the genitive ().

This is a typical Indo-European strategy of a language which lacks both basic region nouns and basic verbs of oriented movement and therefore cannot derive adpositions from them.