It's called a diaeresis. Used in this way, a diaeresis tells the reader that it's pronounced as "co op" and not "coop". http://www.newyorker.com/culture/cultu

It's called a diaeresis. Used in this way, a diaeresis tells the reader that it's pronounced as "co op" and not "coop". http://www.newyorker.com/culture/culture-desk/the-curse-of-the-diaeresis http://www.newyorker.com/culture/culture-desk/the-curse-of-t... Most people would just write "co-op" nowadays -- I've only ever seen this in the New Yorker.