hmm, yep, i use Ms as well.
Ms is anyone female - married or unmarried, much like men. though realistic pershaps more appropriate to those of marriable age.. otherwise i suppose you could have 'Miss' as equivalent to 'Master'.
for the same reasons are previously mentioned.. my marital status is irrelevant. & i may keep my maiden name professionally even if i do marry, so not to complicate things..
not the definitive source.. but from dictionary.com - as obvious as any..lol
Ms. also Ms ( P ) Pronunciation Key (mz)
n. pl. Mses., also Mses also Mss. or Mss (mzz)
Used as a courtesy title before the surname or full name of a woman or girl: Ms. Doe; Ms. Jane Doe.
Used in informal titles for a woman to indicate the epitomizing of an attribute or activity: Ms. Fashionable; Ms. Volleyball.
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[Blend of Miss, and Mrs..]
Usage Note: Many of us think of Ms. or Ms as a fairly recent invention of the women's movement, but in fact the term was first suggested as a convenience to writers of business letters by such publications as the Bulletin of the American Business Writing Association (1951) and The Simplified Letter, issued by the National Office Management Association (1952). Ms. is now widely used in both professional and social contexts. As a courtesy title Ms. serves exactly the same function that Mr. does for men, and like Mr. it may be used with a last name alone or with a full name. Furthermore, Ms. is correct regardless of a woman's marital status, thus relegating that information to the realm of private life, where many feel it belongs anyway. Some women prefer Miss or Mrs., however, and courtesy requires that their wishes be respected.