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 teacher would be offended if you called her “Professor,” whether or not she is one. “Doctor” is usually reserved for medical doctors, although some professors use it, and PhD’s 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.”)