Now:
Ms. The female equivalent of Mr. where marriage is not a defining characteristic
Interesting idea, though personally (but notice I am not from, or living, in USA), even in professional environment, I stick to the language rule, wich was more simply that Ms. is the abbreviation for Miss, and Mr. is the abbreviation of Mister, and the difference between Miss and Mrs is almost the one you indicated.
I say "almost", because in certain environment/context, I know Mrs would be used as a form of "age respect" towards older women, independently from whether married or not, though depending also on their preference.
Though I am not surprised it went the way of the fraulein
- I don't really speak German, but in German language "fraulein" was for unmarried or young, "frau" was for married or older, I have been told nowadays they are all "frau" because fraulein is considered "sexist".
