Many English adjectives and nouns ending in “-ary” have a close match in French, with the ending -aire instead. “Necessary” becomes nécessaire, “ordinary” becomes ordinaire, and “military” becomes militaire. As with other pairs of this kind, both endings come from the same source, the Latin ending -arius, -aria or -arium, rather than French borrowing directly from English. Latin words in -arius passed into French as -aire and into English as “-ary”, usually by way of French itself. The result is a pattern reliable enough to guess many French words on sight, once the shift from “-ary” to -aire is known.

Below are the main groups of examples.

Everyday adjectives

• Arbitraire – “arbitrary”
• Contraire – “contrary”
• Culinaire – “culinary”
• Exemplaire – “exemplary”
• Extraordinaire – “extraordinary”
• Héréditaire – “hereditary”
• Imaginaire – “imaginary”
• Nécessaire – “necessary”
• Ordinaire – “ordinary”
• Sédentaire – “sedentary”
• Solitaire – “solitary”
• Temporaire – “temporary”
• Volontaire – “voluntary”

People and roles

• Adversaire – “adversary”
• Bénéficiaire – “beneficiary”
• Fonctionnaire – “functionary”
• Mercenaire – “mercenary”
• Missionnaire – “missionary”
• Notaire – “notary”
• Propriétaire – “proprietary”
• Secrétaire – “secretary”
• Vétérinaire – “veterinary”
• Visionnaire – “visionary”

Time and calendar

• Anniversaire – “anniversary”
• Centenaire – “centenary”
• Millénaire – “millenary”

Objects and institutions

• Dictionnaire – “dictionary”
• Itinéraire – “itinerary”
• Salaire – “salary”
• Vocabulaire – “vocabulary”

Politics and administration

• Auxiliaire – “auxiliary”
• Budgétaire – “budgetary”
• Complémentaire – “complementary”
• Disciplinaire – “disciplinary”
• Documentaire – “documentary”
• Monétaire – “monetary”
• Parlementaire – “parliamentary”
• Planétaire – “planetary”
• Révolutionnaire – “revolutionary”
• Sanitaire – “sanitary”
• Supplémentaire – “supplementary”
• Tributaire – “tributary”
• Unitaire – “unitary”

A few points worth noting

Some pairs need care rather than a straight swap. Librairie looks like “library” but means “bookshop”; the French word for “library” is bibliothèque. Honoraire means “honorary” as an adjective, but the plural noun honoraires means “fees”, as paid to a professional such as a doctor or lawyer.

Not every English “-ary” word has a matching French -aire form. “Contemporary” is contemporain in French, not contemporaire, while “customary” is usually habituel or coutumier rather than a word ending in -aire. As with any pattern of this kind, it is a strong guide rather than a fixed rule, and each word is worth checking on its own.

Summary

English words ending in “-ary” often match French words ending in -aire, across everyday adjectives, roles and professions, calendar terms and political or administrative vocabulary. Both endings trace back to the same Latin source rather than one language borrowing the word from the other. A small number of English “-ary” words, such as “contemporary”, do not follow the pattern, and at least one common pair, “library” and librairie, is a false friend rather than a true match.