In a Time magazine essay, "In Praise of the Humble Comma," author Pico Iyer compares the comma to "a flashing yellow light that asks us only to slow down." But when do we need to flash that light, and when is it better to let the sentence ride on through without interruption? Here we'll consider four main guidelines for using commas effectively. But keep in mind that these are only guidelines: there are no unbreakable rules for using commas--or any other marks of punctuation.