Regular plural inflection
Inflection marks the compound as a whole, which implies that compounding is done first and then affixation takes place later. The RHR applies to affixes: the rightmost suffix in a word assigns its properties to the entire word.
¸ scissor-handles, trouser-hangers, pant-liners
Headless compounds
To inflect these compound nouns for plural, we attach the inflection in the standard fashion to the rightmost constituent of the compound.
greenhouses, loudmouths, grown-ups, merry-go-rounds, six-year-olds, forget-me-nots
Left-headed compounds
There is a minority of compounds with left-handed heads. They include nouns which form their plural by adding the plural morpheme to the noun in first position, such as the following:
passers-by, mothers-in-law, lookers-on, sons-in-law
Plural of nonhead constituents
In a number of other compounds their first members carry the plural affix, such as
overseas investor, arms merchant
In these cases, the plural suffix in overseas and arms marks the plurality of the nonhead constituent. In order to pluralize the whole compound noun, we have to attach the plural suffix to the head constituent which is on the right.
overseas investors, arms merchants