Correlative conjunction

A correlative conjunction is a pair of linking words that work together to join two matched elements — 'either… or', 'neither… nor', 'both… and', 'not only… but also' — and the two elements they join must be the same grammatical shape.

The common pairs are either… or, neither… nor, both… and, not only… but also, whether… or, and no sooner… than. What earns them a glossary entry is that they impose the strictest parallelism requirement in English: whatever follows the first half must match, in form, whatever follows the second.

• Broken: She not only rewrote the ending but also the first three chapters. A verb phrase after not only, a bare noun phrase after but also.
• Fixed: She rewrote not only the ending but also the first three chapters.
• Also fixed: She not only rewrote the ending but also cut the prologue.

The repair is nearly always the same move: shift the first half of the pair along until the two branches begin at the same place.

Two smaller notes. Neither takes nor, not or. And not only… but also is a heavy construction — it announces in advance that something impressive is coming, and then has to deliver. In narrative prose a plain and usually reads better. Save the correlative for the sentence where the second item genuinely outranks the first.