A complementary or antithetical expression requires a comma at its beginning and end. Perhaps I should define complementary or antithetical expressions. These are usually groups of words complementing or contrasting a preceding word and combining with that preceding word to join it to a following word. The expression should be enclosed in commas:
This harsh, though at the same time logical, conclusion provoked resentment among those affected.
The most provocative, if not the most important, part of the statement piqued our curiosity.
The committee intends to, and no doubt will, dispose of these complaints once and for all.
Avoid a Grammatical Mistake
When you use these expressions (all good writers do), make certain that the lead-in words and the expression enclosed in commas grammatically fit the words that follow. Many writers make terrible mistakes in this regard. See the following example:
Wrong: Earlier the committee could have, and ultimately did, avoid the issue.
You cannot grammatically join could have with avoid. The grammar breaks down. The auxiliary verb did can join with the base infinitive avoid. But the auxiliary verbs could have require the past participle avoided.
So make certain that the complementary or antithetical expression enclosed in commas or parentheses does not force the use of a following word that will not grammatically join with the words preceding the expression. Here you would fashion a new expression:
Right: Earlier the committee could have avoided, and ultimately did avoid, the issue.