Comma with 'Not Only…But Also'
The Comma
Rulebase Summary
In a 'not only…but (also)' construction, use a comma before 'but' or 'but also' when both the 'not only' beginning and the 'but/but also' ending are full clauses. If either the 'not only' part or the 'but also' part is only a word or a phrase (not a full clause), do not use a comma.
Morson's Rule 84 · p.49
Examples
✗
A. He not only reviewed the documents but also interviewed the witnesses.
✓
A. He not only reviewed the documents, but also interviewed the witnesses.
Both parts are full clauses — comma before 'but also.'
✗
A. The defect was not only visible, but obvious.
✓
A. The defect was not only visible but obvious.
Both parts are single words, not clauses — no comma.
Style Guide References (1)
Morson's
Rule 84
Page 49