_-s_ in the plural.

aid-de-camp
attorney at law
billet-doux
commander in chief
court-martial
cousin-german
father-in-law
knight-errant
hanger-on

NOTE.--Some words ending in _-man_ are not compounds of the English
word _man_, but add _-s_; such as _talisman_, _firman_, _Brahman_,
_German_, _Norman_, _Mussulman_, _Ottoman_.


51. Some groups pluralize both parts of the group; as _man singer_,
_manservant_, _woman servant_, _woman singer_.


[Sidenote: _Two methods in use for names with titles._]

52. As to plurals of names with titles, there is some disagreement
among English writers. The title may be plural, as _the Messrs.
Allen_, _the Drs. Brown_, _the Misses Rich_; or the name may be
pluralized.

The former is perhaps more common in present-day use, though the
latter is often found; for example,--