The Plurals of Compound Nouns.
The plural of a compound noun generally formed by the addition s to the main word of the compound.
E.g.: Dairymaids, brothers-in-law, lookers-on, knights-errant, fishermen, tree-frogs, cow-boys, tom-cats, newsagents ...
Some compound nouns do not take plurals as:
Elbow-room, bed-time, horse-power, bird-seed …
Some take the plural in both words.
E.g.: Men servants, lords-justices …
But many reduplicatives [Reduplicatives = a Lesson to come] do not take a plural form.
Ping-Pong, hurly-burly (confusion) and riff-raff (rabble) are used only in the Singular.
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