Usage and Grammar
Q. Is the word but necessary in a not only/but also construction? A colleague says it’s become acceptable to leave off the conjunction, but to do so creates a comma splice. For example: The dictionary not only provides detailed definitions of words, it also has some great pictures.
A. Although comma splices are increasingly accepted in all kinds of writing, in formal English, when you are joining two clauses, you need either an expressed conjunction or a semicolon. You can read about comma splices here and here and here.