There is a question about how to pluralize acronyms. Often a writer will add an 's' following an apostrophe, as in "PC's". However, Kate Turabian, writing about style in academic writings, allows for an apostrophe to form plural acronyms "only when an abbreviation contains internal periods or both capital and lowercase letters". Turabian would therefore prefer "DVDs" and "URLs" and "Ph.D.'s". The Modern Language Association and American Psychological Association prohibit apostrophes from being used to pluralize acronyms regardless of periods (so "compact discs" would be "CDs" or "C.D.s"), whereas "The New York Times Manual of Style and Usage" requires an apostrophe when pluralizing all abbreviations regardless of periods (preferring "PC's, TV's and VCR's").