Is it really necessary to put a comma before since clause? If I omit the comma before since clause in formal writing, will that be taken as a mistake?

We often use as and since when we want to focus more on the result than the reason. As and since are more formal than because. We usually put a comma before since after the main clause:

  • [result]I hope they've decided to come as [reason]I wanted to hear about their India trip.
  • [result]They're rather expensive, since [reason]they're quite hard to find.

Cambridge Online Dictionary: As, because or since?

We use since as a subordinating conjunction to introduce a subordinate clause. We use it to give a reason for something:

Sean had no reason to take a taxi since his flat was near enough to walk to.

  • Since her husband hated holidays so much, she decided to go on her own.
  • They couldn't deliver the parcel since no one was there to answer the door.

Cambridge Online Dictionary: Since

  • 6
    It's normal (but not essential) to use a comma when "since" introduces a reason adjunct, but not when it introduces a temporal one: "I'll have the fish, since I don't like meat" ~ "I've been feeling lonely since you left".
    – BillJ
    Commented Mar 9, 2017 at 10:25
  • I suspect this question has been answered before. HanChen, take a look at the related questions. If you find one that matches closely with what you want to know, please post it here in a comment. Commented Mar 9, 2017 at 15:14
  • @BillJ - Could you move that to an answer, please? Commented Mar 9, 2017 at 15:15

1 Answer 1


No; or rather, only in exactly the same way as it’s ‘necessary’ to put a comma before an ‘as clause’ like the one in your example.

Formal writing or not, in modern English, at least, most commas are optional.

My limited experience of it is that Cambridge is one of the worst English dictionaries available, which is why almost no-one living in England has heard of it.

Prove this for yourself by comparing anything of which you are not certain to Oxford or Webster, for instance. Not in my but in your view, which is better?

With or without any comma, ‘I hope they’ve decided to come as I wanted to hear about their India trip’ throws up at least three questions which might not be truly important in or of themselves, but as evidence for the reliability of a dictionary, cut about as much mustard as a dead, red herring. At best, that sentence is unnatural; it’s rather clearly not a quoted but an artificially constructed example, and not a good one.

In the same way ‘Sean had no reason to take a taxi since his flat was near enough to walk to’ is wholly comprehensible, but was it supposed to be merely comprehensible, or intended to give a clear and helpful illustration of a specific point, while at the same raising no irrelevant questions?

  • I agree that commas are largely a matter of style, and I agree that one should strive for clarity. Let's start by proofreading our answers. // This answer could be further strengthened by removing the editorial comments, e.g. opinions of various dictionaries. Commented Mar 9, 2017 at 15:13
  • No, commas aren't always optional. (As opposed to: "no commas aren't always optional".)
    – Lawrence
    Commented Mar 9, 2017 at 15:19
  • aparente that would be fine but most of the question is founded on Cambridge's views, which are at best self-contradictory. I see that some won't like it and it remains true that having now been introduced to them, I seriously believe the reason I didn't here about, let alone meet Cambridge dictionaries for the first 50 years of my life is far to do dwith their quality than my experience. Commented Mar 11, 2017 at 21:52

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