When punctuating with a comma, the comma goes before the word with the comma. An apostrophe, on the other hand, indicates that a letter is in place of, that is, replacing the word with the apostrophe. A comma is used to indicate a pause, a short break where the thoughts of the speaker can halt momentarily.

When a comma is required directly after a word, it should be placed after the apostrophe, whatever the circumstance. Some writers, particularly in the case of possessive plurals, make the mistake of putting the comma prior to the apostrophe, as follows: Unlike the other horses,’ his hooves had recently been re-shoed. This is incorrect.

In the US, commas and periods always go inside the quotation marks. In the UK, the tendency is to place them outside, unless they appear in the original quotation, in which case they go inside.