Some conjunctions are used in pairs, they are called correlative conjunctions. They are used to show the relationship between ideas expressed in different parts of a sentence.
Most are coordinating correlatives.
both ... and
either ... or
neither ... nor
not only ... but also
He was not only a scoundrel, but also a cruel man.
We use either…or together when we want to link two positives:-
Either the president or the vice-president will go to the conference. (The president will go to the conference, or the vice-president will go, not both of them though.)
We use neither…nor together when we want to link two negative ideas:-
Neither the president nor the vice-president were able to solve the problem. (The president couldn't solve the probliem and the vice-president couldn't solve it either.)
Some are subordinating correlatives.
if ... then
less ... than
more ... than
so ... that
She was so hungry that she could have eaten a horse.