Consider the following sentence:

In 2013 Tallinn, Estonia, instituted fare-free rides for city residents (becoming the largest city in the world to do so), but car use in Tallinn has only slightly declined: as a 2014 study by the KTH Royal Institute of Technology in Sweden found that car traffic in Tallinn was down less than 3% since the policy was enacted.

My question is regarding the colon between the words "declined" and "as". Would it be appropriate to put a comma in place of the colon? Take, for example, the following sentence:

I received a cake and many gifts, as it has been my 21st birthday.

Why would it be inappropriate to put a comma instead of colon in the first sentence? If it is allowed, why would a colon be preferred over the comma?

  • I think you might get better answers if you edit out your last example. The grammar isn't correct, but if you correct it -- I received a cake and many gifts as it was my birthday -- it doens't fit your question any longer. – S Conroy Aug 6 '18 at 23:46
  • Basically, you've got to be anal-retentive to use a colon. – Hot Licks Aug 7 '18 at 1:39
  • Thanks for demeaning my attention to detail and my regard for syntax. You've made my day a little worse. – Hack Saw Aug 7 '18 at 22:05
  • The short answer is “… declined: as…” should prolly have used a full stop, rather than either a colon or a comma, though that would mean other changes. Don’t you think phrases like should be simplified so their meaning doesn’t get in the way of their structure? – Robbie Goodwin Aug 20 '18 at 0:01

Were I editing the first sentence, it's the word 'as' which would go.

Changing the colon to a comma makes the sentence imply that the study was somehow the cause of the decline it found. It's a very muddy construction.

With the colon, but without the 'as', we get two independent statements linked by the colon indicating the second as amplification of the first.

  • Ah, I see. So just wondering, if 'as' was removed, a semicolon would work as well? – Jacob Foster Aug 7 '18 at 4:57
  • Yeah, it probably could. There is controversy around semicolons due to their in-between nature. In fact, the Oatmeal made a poster about this where he says you ought not use one before a conjunction, but the Chicago Manual of Style says it's acceptable. – Hack Saw Aug 7 '18 at 22:18
  • I agree that the 'as' should go, and I would replace the colon with a full stop as well. However I believe that the 'as' is used incorrectly because it implies that the study caused the persistence in car use rather than merely discovered and reported it. – BoldBen Sep 23 '19 at 8:02

Not only should there not be a colon, but even using a comma is unnecessary. Subordinate clauses that follow a main clause don't require commas.

A subordinate clause can go at the beginning of a sentence or later in a sentence. The only difference is that if it goes at the beginning, you need a comma after the subordinate clause, and if goes later, you don’t need a comma.


Your sentence has two other problems, though:

1) The beginning of your sentence should be written as follows:

"In 2013, Tallinn, Estonia instituted fare-free..."

There should be a comma after the introductory phrase "in 2013," but there should be no comma after "Estonia."

2) It seems that you're using "as" to mean "because," but that study isn't why car use in Talinn has only slightly declined. I think what you're trying to say is:

"...according to a 2014 study by KTH Royal Institute of Technology in Sweden, which found that car traffic in Tallinn was down less than 3%."

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