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