When I encounter an unknown woman (careful, might be dangerous!) I always put heavy stress on the MZZZZ (Ms) in the hopes that she will give me her preference as to Mrs, Miss or first name). If she takes umbrage, fine, that's all part of the war between the sexes. I am amused by Webster's comment on Ms. in their definition. "Probably blend of Miss and Mrs." The horrible word first appears in print in 1949, so it is newspeak. Seriously, the advice of Joe Offer et al., politely to determine preferences is best. And there are regional differences as he pointed out. Foreign women of stature in learning often insist on Madame. In much of the south and west, ma'am is accepted in most primary schools (unless the teacher is male, which is rare at that level). The teacher may request address as Miss or Mrs Jones, but cannot reject ma'am because it has been ingrained by parents. In a university setting, Ph.D. owners are addressed as Dr, but (being one) I use Mr outside that touchy atmosphere. In Canada, however, a Ph. D makes it a point to be referred to as Doctor. Modern English practice is to leave off the period after Miss, Mr, etc., while American practice continues it (I agree with the English practice here). Lots of pitfalls, as the length of this thread suggests, and I have fallen in most of them.