-x

Forming the plural of many nouns ending in -u taken from French.

French plural form.

Some French words ending in ‑eau or ‑eu brought into English have retained their French plural forms: bureaux, châteaux, milieux, plateaux, tableaux. It is increasingly common to see these with the standard English plural ‑s instead (see ‑s¹), especially in American English, though châteaux has been least affected. Some have completed the shift: adieus, portmanteaus, purlieus, trousseaus. See also ‑trix.