nona the brit“If you don't know. use Ms. as it is neutral regarding marital status.
Miss is for unmarried women.
Mrs is for married women.
Ms is for 'unknowns' and any woman who wishes to use it.”
This is a good short answer. MrMicawber, Clive and Grammar Geek have sound advice too. Here's my two cents:
If the marital status is unknown, use Ms. If the woman's preference is unknown, use Ms. If the woman prefers Ms., use it. (And it is definitely pronounced Mizz.)
If you know the woman is married, use the title the woman prefers if you know her preference. If you don't know the preference, there is a judgment call. If it is a business situation, I would use Ms. If it is a teacher, I would use Ms. (I remember in school, several teachers did not make it clear and we mumbled something halfway in-between.) For casual acquaintances, I would use Ms. However, with my childhood friends' mothers, I still use Mrs. It just feels right. I've never asked their preference and after so many years it would be awkward.
Also, someone mentioned calling their friend's mothers by their first names. I do for some, when I met them as an adult, but I just can't imagine calling my best friend's mother Catherine. I've known her since I was six. She has always been and will always be Mrs. X to me.
Names are given but also are a personal choice. My wife took my last name. She uses Mrs. Her First Name Our Last Name. However, she frequently gets mail addressed Ms. She takes no offense; she knows they don't know she is married or don't know she prefers Mrs.
My sister-in-law had an established career when she married my brother. She kept her maiden name. She uses Mrs. Her Maiden Name.
I know other women that prefer Ms.
In business, I don't think you can ever go wrong with Ms.
Miss is not used much anymore. I don't know any women that prefer to be called Miss. Here in the South, it is still used as a term of respect or endearment in the form: Miss First Name. It isn't that common and don't do it with someone that isn't family or a close friend. The movie Driving Miss Daisy is a good example of this use. I'll call my nieces Miss. First Name, albeit somewhat sarcastically. Mothers will use it when angry at their daughters. Miss First Name get in here right now!