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Is it two-third or two-thirds? If both are correct, how does their usage differ?

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possible duplicate of Is it proper to use ordinal suffixes on fractions? –  Mari-Lou A Jul 29 '13 at 17:12
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Any fractional number (half, third, fourth, ...) is singular following one, and plural following any other cardinal number. –  John Lawler Jul 29 '13 at 17:16
    
Referring to the comment from @JohnLawler, note that Americans tend to use a fourth, whereas Brits use a quarter. I think it would be extremely rare to hear a Brit using a fourth. –  TrevorD Jul 29 '13 at 18:51
    
Quarter is used by itself to mean 1/4 in the US (only a quarter tank left), but it's also the name of the US 25¢ coin. 3/4 can be either three-fourths or three quarters. Fourth generally requires of plus an article, while singular quarter doesn't: a/one fourth of a mile, a/one quarter (of a) mile, three-fourths of a mile, but three quarters of a mile. –  John Lawler Jul 29 '13 at 19:16
    
@JohnLawler Interesting. I think we (Brits) would normally say "a quarter of a mile", altho' we may write "a quarter mile". I don't think we would normally say or write "one quarter (of a) mile". –  TrevorD Jul 29 '13 at 23:04

1 Answer 1

A third, like any other unit of measurement, is dependent on the number of units described. For example, we describe 'one apple' and 'two apples'. The same goes for fractional units. 'one third' is simply a single unit of 'third'. Two or more and we use 'thirds'; hence, 'two-thirds' is the correct usage.

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