A few plural nouns refer to things that have two parts. They have no singular form. These are always things we wear:
Those trousers are too long.
These binoculars were very expensive.
To make it clear we are talking about one of these items, we use a pair of …:
I need a new pair of spectacles.
I've bought a pair of blue jeans.
If we want to talk about more than one, we use pairs of …:
We've got three pairs of scissors, but they are all blunt.
I always carry two pairs of binoculars.
- Singular, plural or both?