Ms. is the not-so-new address that is agnostic about the married status of the woman.
Many people find it demeaning to address women differently depending on whether or not they are married, especially when we don't do the same for men.
That's funny, I've met men who think it isn't fair and wish we got a title change too!
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>especially when we don't do the same for men.
I was always taught it was 'Master' for unmarried and young men.