3
  1. A double number is two identical numbers, e.g. 007 (double O seven)
  2. A double chin is someone who appears to have two chins
  3. A double feature is a program showing two feature-length movies
  4. A double D is a bra with two DD cups
  5. A double standard is a rule or principle which is unfairly applied in different ways to different people or groups (EOD)
  6. A double yolk is an egg with two yolks

All the nouns listed above are countable but stay singular. On the other hand, if we substituted the word double with two the noun is pluralized e.g.

I cracked open an egg and found two yolks

Why don't we use the plural noun in these cases?

  • I cracked open an egg and found double yolks (NO)
  • I cracked open an egg and found a double yolk (YES)

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There appears to be an exception to the rule and it is double doors. According to Ngram, today the plural form compared to its singular counterpart is used 80 times more frequently.
(Hopefully, I didn't mess up the maths, but if I did please correct me)

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3

According to Collins dictionary, double as defined in definition 2:

Adj. You use double before a singular noun to refer to two things of the same type that occur together, or that are connected in some way.

E.g ...an extremely nasty double murder.

2

Because the subject of is in your examples is the name for something. That thing is understood as a single compound entity or thing.

I bought tickets for the double-header but it got rained out.

  • But the double yolks are inside the egg (?!) And in the sentences the subject is "I" and "a double yolk" is the object. – Mari-Lou A Nov 12 '18 at 16:34
  • I am talking about your list of sentences at the top. We don't say She are a triple threat. The reference is to a monad of three. She is a triple threat. A triple threat is what she is. – Tᴚoɯɐuo Nov 12 '18 at 17:47
  • +1 for "monad" I had to look it up, I haven't seen that word in years – Mari-Lou A Nov 12 '18 at 18:21
  • Monads are crepuscular. But you've probably seen plenty of dyads and triads, tetrads and pentads in broad daylight. – Tᴚoɯɐuo Nov 12 '18 at 18:33

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