Correlative conjunctions always appear in pairs—you use them to link equivalent sentence elements. The most common correlative conjunctions are “as…as,” “just as…so,” “both...and,” “either...or,” “neither...nor,” “not only...but also,” “not…but,” “so...as,” and “whether...or.” (Technically correlative conjunctions consist simply of a co-ordinating conjunction linked to an adjective or adverb.)

The highlighted words in the following sentences are correlative conjunctions:

Both my grandfather and my father worked in the steel plant.

In this sentence, the correlative conjunction "both...and" is used to link the two noun phrases that act as the compound subject of the sentence: "my grandfather" and "my father".

Bring either a Jello salad or a potato scallop.

Here the correlative conjunction "either...or" links two noun phrases: "a Jello salad" and "a potato scallop."

Corinne is trying to decide whether to go to medical school or to go to law school.

Similarly, the correlative conjunction "whether ... or" links the two infinitive phrases "to go to medical school" and "to go to law school."

The explosion destroyed not only the school but also the neighbouring pub.

In this example the correlative conjunction "not only ... but also" links the two noun phrases ("the school" and "neighbouring pub") which act as direct objects.

Note: some words which appear as conjunctions can also appear as prepositions or as adverbs.