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

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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
@Mari-LouA I don't think this is a duplicate of the linked question: that one is discussing using digits or words, whereas this one is asking about using the plural forms. –  TrevorD Jul 29 '13 at 23:06

3 Answers 3

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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Quite correct, but it should be noted that the hyphen is not always used:

The motion was carried by a two-thirds majority.

At least two thirds of business research and development expenditure was found to be wasted.

depending on whether the phrase is used as an attributive adjective (first case) or simply as an adective + noun combination (second).

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In the second case, whether or not to use a hyphen can be a matter of style. For example, The Guardian's house style is to always use a hyphen. –  George Stirling Nov 14 '14 at 13:28
I don't agree that it's a style issue. Most authoritative references on the English language consider that when used as a non-attributive adjective, the hyphen should not be present (Oxford Dictionary, Chicago Manual of Style, and more). Since when has the Guardian been such a reference? "One half of the beach was covered in oil." Would it be OK to use a hyphen here? –  thelonius Dec 16 '14 at 8:33

Any fractional number (half, third, fourth, ...) is singular following one, and plural following any other cardinal number. – John Lawler

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