A NINE FOOT PIANO OR A NINE FEET PIANO ?

ANSWER: With different measurements of time/distance/length it is correct to use the plural when the description comes after the noun and to use the singular when the description comes before the noun.
Example:
A 9 foot piano. The piano is 9 feet long.
A seventeen year old youth. The youth is seventeen years old.
A 100 metre high building. A building which is 100 metres high.
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