They turned to a much older dating method: dendrochronology, the use of tree rings for mapping intervals of time. (From ACT English)

Why in this case has it to be a colon, but not a comma? I think since "dendrochronology" is in apposition with "a much older dating," both punctuations should be acceptable.

  • Note that the bit after the comma is the definition of dendochronology, so is part of the same phrase; putting a comma before as well would make that harder to read. Nov 8, 2020 at 10:20
  • @Daniel Roseman But adding a comma would be grammatically correct nevertheless? Nov 8, 2020 at 13:02
  • [Basic English: Does it have to be, not has it to be]
    – Lambie
    May 27 at 22:35

2 Answers 2


One of the functions of a colon is to introduce something that answers or illustrates what precedes it.

I'll tell you how I knew that: I looked it up.

Then I realized what I needed: air.

Here's what I do: I play golf

deadrat's answer to a similar question may be helpful.


A colon is use to provide explanation or example of the sentence that precedes it.

In the above example, we are providing example of an older dating method. Therefore, a colon has been used, whereas a comma is use to list things or for connecting two complete sentences using a linking word.


I bought vegetables, beverages and fruits. (listing things)

I went to supermarket, and I bought vegetables, beverages and fruits.

(connecting two complete sentences)

for reference:



  • I mean comma can also be used to introduce appositions. In this case, I think "dendrochronology" is an appositive of "a much older dating" Nov 8, 2020 at 7:55
  • 1
    @Charlie Yes: we could drop the colon and treat "dendrochronology" as a supplementary appositive of "much older dating method". We can have "They turned to dendrochronology, the use of tree rings for mapping intervals of time". Note that supplementary appositives are not modifiers.
    – BillJ
    Nov 8, 2020 at 10:30
  • @ BillJ But the thing is, this is actually a multiple choice question where the only right answer is with the colon, not comma. Nov 8, 2020 at 12:15

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