I'm having trouble knowing which to use. i have of what I think are gray areas, so I'm trolling for opinions. Case 1: A formal marine sergeant addressing a civilian 17 year old girl while in the official commission of his duties and affording her the same level of respect as he would the base commander's daughter. He's known her for several years so he settled on an address with she was 14 or 15 and may or may not have updated his form of address. It is also in front of his privates so he has to set the right tone. I suspect if he was addressing her directly he might just call her ma'am, despite it being wildly incorrect, but when referring to her to a third party, that doesn't really work. “PFC Winters, see Ms./Miss Winborne to her quarters.” It seems as though Miss is generally used until a woman is about 18, so she's right on the cusp. I'm not sure how the military's emphasis on showing respect and titles would play into this. So any opinions on what title the Sergeant would use? Case2: When my friends with kids introduce adult women to them they are often introduced as miss/ms. (not sure which) first name. Teachers are often referred to Ms./Miss surname. So the aforementioned Ms./Miss Winborne has a teacher, legal guardian and surrogate mother whom she internally refers to as Ms./Miss Amy. By etiquette reasoning she is old enough that she shouldn't be called Miss anymore, however in my research Miss is apparently often used with teachers regardless of age or marital status. The choice would have been made while the girl was about 10 and Amy was in her late 20's if it makes a difference. So is it Miss Amy or Ms. Amy?