NUMBER.
Certain compounds change the form of the first word in pluralizing, as: court-martial, brother-in-law, sister-in-law. Plural, courts-martial, brothers-in-law, etc. "John has three brother-in-laws," then, is incorrect.
But tea-spoonful, table-spoonful, cupful, pocketful, etc., are not considered such compounds; therefore, "two tea-spoonsful of medicine" and "two-cupsful of flour," should be, "two tea-spoonfuls of medicine," and "two cupfuls of flour."
When name and title are given, with a numeral adjective prefixed, the name is pluralized. "Are the two Misses Wilson at home?" should be, "Are the two Miss Wilsons at home?" But when the numeral is omitted the title must be pluralized. "Were the Dr. Browns there?" should be, "Were the Drs. Brown there?" The rule has been given that the name only of married ladies is pluralized, but there appears to be no reason except that of euphony: the Mrs. Clarks certainly sounds more agreeably than the Mistresses Clark. In giving the plural of such titles as: Hon., Rev., Squire and Capt., euphony is also often considered; but in such cases it would doubtless be better to add the numeral, as: the three Hon. Jacksons.