There were eight Henrys, kings of England.
The two Marys reigned in the kingdom.
It would be confusing to say eight Henries, the two Maries.
The title is made plural when several are referred to, thus:
78. The title is made plural when used with several names, thus:
Messrs. Brown and White.
Generals Lee and Grant.
Drs. Long and Larson.
79. In the case of nouns formed of two or more words, when the compound word is so familiar that the parts are not thought of separately the s is added to the whole compound word, as four-in-hands; forget-me-nots; court-yards; spoonfuls; green-houses; etc. But when one of the parts is more important than the others, the s is added to the more important part, thus:
mothers-in-law
commanders-in-chief
hangers-on
men-of-war
by-standers
attorneys-at-law
passers-by
step-sons
80. We have many words in our language taken from other languages. They do not form the plural in these languages as we do, and some of these words retain their foreign plurals. Some of the most commonly used of these nouns are the following:
81. The following nouns are treated as singular: news, pains (meaning care), acoustics, mathematics, economics, ethics, molasses, physics, politics, and other nouns ending in ics except athletics. With these always use the s-form of the verb. For example:
The news is distorted. Not, The news are distorted.
Economics is an important study. Not, Economics are, etc.
82. The following nouns are always plural:
alms
annals
amends
antipodes
bellows
billiards
clothes
dregs
eaves
fireworks
hysterics
measles
mumps
matins
nippers
nuptials
oats
premises
proceeds
pincers
riches
rickets
suds
scissors
thanks
tidings
tongs
trousers