Adpositions constitute a word class that only appears either before or after a complement (noun, noun phrase, pronoun, or clause serving as a noun phrase). Appearing before a complement they are prepositions; appearing after they are postpositions. The function of adpositions is to express the relationship between the complement and another noun or verb phrase, as well as express semantic relations such as place, time, quality, instrument, and possession. Adpositions are heads, and the nouns or noun phrases that they modify are the complements or objects of the adpositions.
In certain languages, adpositions that modify non-personal pronouns can express person marking. For this to occur, two criteria must be fulfilled: a) the forms must show sufficient differentiation (including zero morphemes) for the first, second, and third persons (or at least two of three) in order to show contrast; b) marking must use affixes and not clitics.
Types (PM – person marker, PP – non-personal pronoun):
NoAdp: The language does not have adpositions.
AdpNonPM: Person marking cannot be expressed on adpositions.
AdpPPNonPM: Person marking cannot be expressed on non-personal pronominal adpositions.
AdpP(PM): Person marking is optional on non-personal pronominal adpositions.
AdpPPM: Person marking is required on non-personal pronominal adpositions.
This function can be served not only by adpositions but also lexically or syntactically (e.g., conjugated verbs or declined nouns, case endings, adverbial affixes, and clitics). Some of these can be derived from adpositions, but as bound forms they cannot be considered true adpositions or heads. Nor do they have their own person marking. The adposition is therefore a separate word class; it is morphologically independent and shows morphosyntactic behavior in which in the given language it clearly differs from that of verbs, (declined) nouns, and adverbs.
 Clitics are distinguished from affixes primarily by the flexibility of their attachment within an adpositional phrase, as they can attach to either or both constituent(s) of the phrase: the noun/pronoun and the adposition.
 This parameter value will automatically apply for all languages with the value NoAdp for the parameter Word order of adpositions.
 With this value, it is assumed that person marking do occur on adpositions alongside nouns and/or personal pronouns.