Things with two parts

A few plural nouns refer to things that have two parts.
They have no singular form. These are always things we wear:

Things we wear

spectacles

pyjamas

tights

trousers

pants

jeans

shorts

boxers

knickers

Example
Those trousers are too long.

Or implements

pliers

scissors

binoculars

pincers

Example
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.