Q. If someone has a PhD and is a professor at a university, what would be his or her title? Doctor or Professor?
A. Although this question doesn’t really fall within the purview of CMOS, the manuscript editing department at Chicago is of course well versed in etiquette, as we are in most things. Traditions vary from school to school and from discipline to discipline. You’re always safe with Mr. or Ms., but I doubt that any teachers would be offended if you called them Professor, whether or not they are one. Doctor is usually reserved for medical doctors, although some professors use it, and PhDs who don’t have tenure-track appointments (and who therefore don’t hold the title of professor) often like to use Doctor instead. (For other questions of etiquette, you can browse the Internet for “etiquette” or “manners” or, in this case, “forms of address.”)