Okay, which do you think would make a better book title? Aristophanes' Knees? Or Aristophanes's Sneezes? According to most grammar mavens, "ancient" names like Aristophanes (along with Sophocles, Pericles, Euripedes, Antigones, or even Jesus, Moses, etc.) should not get an apostrophe s in the possessive form, but rather just a plain apostrophe. This "rule" is based on the way it sounds when you say the word out loud. But I don't think it really has anything to do with the antiquity of the name; I believe you should apply it in all cases where, were you to add an apostrophe s, it would sound sort of like a stutter or a sneeze. (Awkward, as the kids would say.) In other words, take any name that ends in an s and try saying it as a possessive. Then add either an apostrophe or an apostrophe s, whichever you think sounds better. In some cases, you'll probably find that it rolls off the tongue easier without that last messy syllable. (The more syllables a word has, and the more the stress occurs toward the beginning of the word, the more this seems to be the case. Some guides also make a distinction between names that sound like they end with a z and ones that sound like they end with an s.) Writing, of course, is different from speaking, and here you can often opt to do it either way, bearing in mind that however you write it is most likely how your reader will hear it. You are now in possession of a rather complicated grammar rule, which can be made a lot simpler by simply adding an apostrophe s to any proper name in the possessive form, no matter what it is. There were 52 instances of this typo in OhioLINK today, and 1259 in WorldCat.