No Comma Before an Essential Adverbial Clause
The Comma
Rulebase Summary
Do not use a comma before an adverbial clause that is essential to the meaning of the sentence. The clause is necessary — removing it would change the core meaning. Adverbs that frequently introduce essential clauses include as, because, if, since, so (purpose), and unless. The same word can introduce essential or nonessential clauses depending on context.
Morson's Rule 65 · p.41–42 BGGP (Al)though.1 · p.405 BGGP So.1 · p.418
Examples
✗
A. I called 911, because I heard glass breaking.
✓
A. I called 911 because I heard glass breaking.
Essential 'because' clause — no comma before it.
✗
A. He was terminated, if he refused to comply with the policy.
✓
A. He was terminated if he refused to comply with the policy.
Essential 'if' clause completes the meaning — no comma.
Style Guide References (3)
Morson's
Rule 65
Page 41–42
BGGP
(Al)though.1
Page 405
BGGP
So.1
Page 418