You can use eight-foot boards to side a house, but “foot” conveys a plural sense only in this sort of adjectival phrase combined with a number (and usually hyphenated). The boards are eight feet (not foot) long. It’s always x feet per second and x feet away.
This is the ten-year anniversary of the Common Errors in English Usage calendar. To celebrate, we are bringing back some of our favorite interesting, funny, but sometimes merely silly entries through the years before going on hiatus in 2016.
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