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 their complement and another noun or verb phrase, as well as express semantic relations such as place, time, ownership, instrument, and possession.[1] Adpositions are heads, and the nouns or noun phrases that they modify are the complements or objects of the adpositions.

In certain languages, nominal adpositions (i.e. adpositions with nouns (N) as their objects) 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.[2]

Types:

NoAdp: The language does not have adpositions.[3]

AdpNonPM: Person marking cannot be expressed on adpositions.

AdpNNonPM: Person marking cannot be expressed on nominal adpositions.[4]

AdpN(PM):  Person marking is optional for adpositions when they appear with nouns.

AdpNPM: Person marking is required for adpositions when they appear with nouns.

[1] 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 adposition or heads. Nor do they have their own person marking. (Consider the phrase ‘in my house’ in Hungarian and Finnish, respectively: ház-am-ban (house-Px.S1- INE) and the Finnish talo-ssa-ni (house-INE-Px.S1). Despite the different morpheme order, the personal affix codes the noun, not case, in contrast to the postpositions in these languages that bear true 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.

[2] Clitics are distinguished from affixes primarily by the flexibility of their attachment; for example, they can attach to nouns and/or adpositions alike.

[3] This parameter value will automatically apply for all languages whose value for “Word order of adposition” is NoAdp.

[4] With this value, it is assumed that person marking does occur on adpositions when they appear alongside pronouns (personal and/or non-personal).