The suffix -air originates from French. It is stress-bearing, and cohering, as syllabification does not respect morphological structure.

Adjectives in -air have a relational meaning (cf. (Booij 2002: 108ff)), i.e., their meaning can often be described as having something to do with the meaning of the base. There is a division of labor with (equally nonproductive -oir) ((De Haas and Trommelen 1993: 336ff)).

(De Haas and Trommelen 1993: 213) distinguish three types of nouns in -air:

• personal nouns, based on non-native nouns and roots such as diamantairdiamond seller (< diamantdiamond), miljonairmillionaire (< miljoenmillion), antiquairantiquarian (antiekantique(s)), stagiairintern (stageinternship). These forms are of common gender, taking the indefinite singular article de, and have a plural form in -s. Some can have a feminine counterpart in -e, e.g. diamantairefemale diamond seller, stagiairefemale intern.
• personal nouns, based on non-native nouns and roots such as visionairvisionary (< visioenvision) and militairsoldier, militairy (possibly from a root *milit, cf. below). They are of common gender, taking the indefinite singular article de. Some of these have a plural form in -s, e.g. veterinairsvets, others in -en, e.g. militairensoldiers. There is no morphologically marked feminine counterpart.
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  (De Haas and Trommelen 1993: 213) suggest that the nouns of this second class may be cases of conversion of the homophonous adjectives. A radically different perspective is taken by (De Belder 2011): working within the framework of Distributed Morphology, De Belder opines that these formations have no syntactic category in the lexicon and only get one in the syntactic process of sentence formation.

• two nouns in -air denote collections of things, viz., sanitairsanitary facilities, plumbing and meubilairfurniture. These nouns are of neuter gender, taking the singular indefinite article het, and do not have a plural form.
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  According to (De Haas and Trommelen 1993: 213), these nouns can also occur as adjectives, but for many native speakers this does not hold for meubilair.

Morphological potential: words in -air can be input to noun formation in -iteit (polariteitpolarity < polairpolar) or -isme (militarismemilitarism < militairmilitary), or verb formation in -iseer (populariserenpopularize < populairpopular). Note that the vowel ai/ε:/ changes into a/ɑ:/, an instance of learned vowel backing (Dell and Selkirk 1978, Booij 1995: 77, cf. suffix allomorphy).

Many cases of -air formation show phonological or orthographical changes in the base word, e.g. prevocalic deletion of schwa in stagiarintern (< stage) and disciplinairdisciplinary (< disciplinediscipline) or vowel change in circulaircircular (< cirkelcircle).

Many formations in -air are based on roots that do not occur as independent words. In cases such as militairmilitary one might think of affix substitution, as there are other forms based on the same root, e.g. militia. Probably, most Dutch -air formations are loans. The fact that /ε:/, the vowel in the suffix, does not occur in native words is additional evidence for this hypothesis.