Let’s take a look at the following sentence:
Bad Example (but good news):
Oil production has increased during this six-months period.
As with all critiques, we will start off by saying something positive: the hyphen definitely belongs there. However, the “S” at the end of “months” does not.
“Why?” you may ask. “There are six months, and that is more than one, so it should be plural, right?”
No. The reason is that the compound adjective, six-month, is an adjective, and in the English language adjectives do not have plurals.
Only when “month” is standing alone and functioning as a noun does it become plural.
Oil production has increased during this six-month period. (compound adjective)
Oil production has increased during these six months. (noun)
Other good examples of singular compound adjectives:
Eight-piece jazz band
Nine-digit account number
Post-Election Day Quote:
“Ballots are the rightful and peaceful successors to bullets.”
– Abraham Lincoln, 16th President of the United States, 1809–1865