The comma is a punctuation mark that appears in several variants in different languages. It has the same shape as an apostrophe ( ' ) or single closing quotation mark in many typefaces, but it differs from them in being placed on the baseline of the text. Some typefaces render it as a small line, slightly curved or straight but inclined from the vertical, or with the appearance of a small, filled-in.
This is a companion discussion topic for the original entry at http://grammarerror.com/tags/comma