Maybe I should have looked in the OED earlier. I'm assuming I can use "Ms" since it is "a title prefixed to the surname of a woman, regardless of her marital status" (so much for the need to explain...). Someone once told me "Ms" only applied to divorced women -- always had a funny feeling about it. Although I suppose it does make more sense for divorced women than for others. Sorry folks, I think the OED has just satisfactorily answered my question. Unless, of course, there is someone who knows better...?

North Amer. - Ms. seems to be the safest form and commonly used to address any woman no matter what her marital status unless she has requested one of the other forms. Actually, a lot of "official" documents (forms) now ask for preference of address, Mrs., Miss or Ms.

A good question, Doris. As for written English this is clear to me now. How do you address someone in verbal speech, though? Do you say "Miss" to any woman no matter what her status?

I think people say "Ms" too, pronounced m–schwa–z

N. Am. - people do say Ms. (pronounce Miz with a long i).

I think that the use of the form Ms [mi:z] came up around the time when in Germany they stopped to make use of "Fräulein" for unmarried women. The intention of it for me is clearly not to discriminate women because of their marital status. I believe this change came up around 1980 or a little bit later.

The use of "Ms." in the United States goes back to the feminist movement of the 1960s and 70s. The term itself was accused by some as being "politically correct" and although it has moved into the mainstream in most areas, some socially conservative people refuse to use the term as having excessively P.C. or too-liberal overtones. Some women in that category refuse to be called by that term, but for the most part it has gained wide acceptance.
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Pronounce as if written "mizz".
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The term was well enough known by 1971 for it to be used as the name of a new feminist magazine called "Ms.", which still exists now.
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More info on the magazine at
<br>http://www.msmagazine.com/msherstory.html and probably plenty of links on that site to feminist history (they say "herstory" a bit tongue in cheek--it's definitely not an accepted term by the mainstream) which may shed further light on the origin of the term.

In speech, I would use Mizz when addressing someone if I didn't know.

Also, Ms. doesn't have to mean that the person is devorced; many women prefer their old Mrs. XYZ name, even after they are devorced.

It's spelled 'divorce'. I just had to clear that up even though this thread is years old!