Irregular Plurals in French

Explanation: "Hibou" is one of the 7 French words ending in -ou that take an X in the plural: bijou, caillou, chou, genou, hibou, joujou, pou.

Explanation: "Cheval" becomes "chevaux" in the plural, like most words ending in -al (journal/journaux, animal/animaux). Exceptions: bal, carnaval, festival...

Explanation: Œil becomes yeux in the plural, a transformation inherited from Latin "oculus" (plural "oculi"). Exception: "des œils-de-bœuf" is used for round windows.

Explanation: "Travail" becomes "travaux" in the plural. Note: "des travails" exists only to refer to the apparatus used by farriers.

Explanation: Only 7 French words ending in -ou take an X in the plural: bijou, caillou, chou, genou, hibou, joujou, pou. All others take an S (clous, trous...).

Explanation: "Festival" becomes "festivals" in the plural — it is one of the exceptions to -al words that usually become -aux (bal, carnaval, chacal, festival, récital...).

Explanation: "Monsieur" becomes "messieurs" in the plural, because the word comes from "mon sieur" (my lord), and "sieur" became "sieurs" in the plural.

Explanation: "Vitrail" becomes "vitraux" in the plural, like most words ending in -ail (travail/travaux, émail/émaux). Exceptions: détails, rails, portails...

Explanation: "Ciel" becomes "cieux" when referring to the celestial vault. "Ciels" exists for artistic representations (Turner's skies).

Explanation: "Aïeul" becomes "aïeux" when talking about ancestors in general. "Aïeuls" specifically refers to grandparents (the four grandparents).