1.9.4 Plural noun as premodifier

Rule: Plural nouns usually become singular when modifying other nouns.

Examples of errors:

1. In a two million inhabitants town you never meet anybody you know.
2. The orphanage was full of five to seven-years-olds.
3. That's a twenty dollars dress.

Rule 1.9.4 holds for most nouns, including those that normally have no singular form:

a race for horses = a horse race
a chair with arms = an arm chair
a leg of a pair of trousers = a trouser leg
a sharpener for scissors = a scissor sharpener
a board for darts@= a dart board

There are exceptions, however:

a depot for arms (i.e. weapons) = an arms depot
a committee on elections = an elections committee
a tax on sales = a sales tax