According to The American Heritage Book of English Usage , Using Ms. obviates the need for the guesswork involved in figuring out whether to address someone as Mrs. or Miss: you can’t go wrong with Ms. Whether the woman you are addressing is married or unmarried, has changed her name or not, Ms. is always correct.
And the beauty of Ms. is that this information becomes irrelevant, as it should be — and as it has always been for men.
english.stackexchange.com/users/300/RegDwigнt
RegDwight has given a correct modern answer.
The traditional approach would be different.
Adopting this approach will sound like you're living in the 1950s or earlier, so you shouldn't actually do this, but I thought it would be useful to document what the modern system is reacting against.
You should always know whether a woman is married or unmarried; a woman will sign a letter as either Rachel Jones [unmarried] or Rachel Smith (Mrs.) [married].
Other than in correspondence, you would have been introduced, or there would be a calling card.
If you write to a married or widowed women ( divorced women are excluded from polite company)
then you address her in writing as Mrs. Stephen Smith, using her husband's first name, not her own.
For this reason you can write to "Mr. and Mrs. S. Smith".
Mrs. and Miss are titles of respect; you only use them in relation to ladies; the lower classes are addressed by their first names, or by a bare surname.
This was the usual approach before 1960 or so (after the separation of Mrs. and Miss from the original Mistress in the seventeenth century).
The use of Ms. (at all) was controversial from its proposal by Sheila Michaels in 1961 until some point in the 1980s - William Safire's use of it for Geraldine Ferraro in 1984 is often seen as the pivotal moment for acceptance by its former opponents, in that someone as traditionalist as Safire realised that he couldn't use anything else.
I believe that the announcement of the engagement of "Prince Henry of Wales and Ms. Meghan Markle" on 27 November 2017 is the definitive proof that the title is appropriate in the highest of society.
If you're writing historical fiction, or you want a character to sound very old-fashioned, then using the traditional forms would be appropriate; even if not, it's useful to know what they were.
english.stackexchange.com/users/1265/Richard Gadsden
(This question has been referenced from another marked as a duplicate, so I'm adding an answer here even though this question is old.)
The catch is that this is all politically/socially charged.
The title "Ms" was invented in the 70s (or at least that's when it became widely known) by feminists who objected to being "identified based purely on their relationship with men", to use a phrase that was commonly repeated at the time.
Their intent was that Miss and Mrs should be abolished and replaced with this new "non-sexist" term.
But because the term was advanced as part of a specific philosophy or social agenda, women who did not support that agenda objected to having the term applied to them.
So what we really ended up with was that feminist women identified themselves as "Ms", unmarried non-feminist women identified themselves as "Miss", and married non-feminist women identified themselves as "Mrs".
What could under other circumstances have simplified titles, reducing 2 to 1, resulted instead in complicating them, increasing 2 to 3.
I don't think there's as much heat attached to it today as there was back then.
But you can't just say, "Oh, it's easier to use Ms for all women because then I don't have to know whether or not they're married."
Many non-feminist women don't like the title and object to having it applied to them, just as many feminist women object to the titles Miss and Mrs.
The only "easy answer" is to call a woman by the title she prefers.
If she refers to herself as "Mrs Mary Smith", then that's what you should call her.
If you don't have anything from her giving a title, ask her.
Well, I suppose if you want to make a social point, for or against feminism, you could use the title you prefer whether she likes it or not.
Personally, I just avoid using any title at all when I can.
I just refer to her as "Mary Smith" without any Miss, Mrs, or Ms.
english.stackexchange.com/users/13140/Jay
Assuming that I do not know if the individual addressed is married, when should I use Miss Brown, and when should I use Ms. Brown?
You might use "Miss" to address a female child under the age of thirteen.
Otherwise, if you don't know if they are married, then Ms. is a convenient get-out.
english.stackexchange.com/users/None/
A rather strange point was raised in the accepted answer to this question.
Personally, I wish this information were relevant for both men and women, but the fact is, it is a feature of our language that men are always styled 'Mr' unless they have some other title or style, and women have the privilege of being styled 'Miss' or 'Mrs'.
Some women will be seriously offended if you don't address them by their preferred title.
In business correspondence, 'Ms' is almost always appropriate, but in social correspondence, one would do well to try to find out what the lady in question prefers.
english.stackexchange.com/users/1065/J D OConal
There has been some disagreement in my other online searches, and in my own education.  Assuming that I do  not  know if the individual addressed is married, when should I use  Miss Brown , and when should I use  Ms. Brown ?
When is it appropriate to use the title "Miss" as opposed to "Ms."?
english.stackexchange.com
<greetings><honorifics>
english.stackexchange.com/questions/2743
english.stackexchange.com/users/1115/Patrick Johnmeyer
There is no agreement about what the most appropriate titles are for addressing a woman whose marital status is unknown. Ms. is often recommended as a good option, especially in business correspondence, although this may cause offence to people who prefer to be referred to as Miss or Mrs. In general, it is probably best to avoid the title altogether and simply use her full name. Alternatively, you could try to find out which title she prefers. Ms. applies to both married and unmarried women so you will never wrongly address a married woman as Miss or vice versa but some women dislike Ms so you should use whatever form of address they prefer or none. 
There is no agreement about what the most appropriate titles are for addressing a woman whose marital status is unknown. Ms. is often recommended as a good option, especially in business correspondence, although this may cause offence to people who prefer to be referred to as Miss or Mrs. In general, it is probably best to avoid the title altogether and simply use her full name. Alternatively, you could try to find out which title she prefers. The title "Ms" started to be used in the 1970s by feminists who objected to being "identified based purely on their relationship with men". If you're writing historical fiction, or you want a character to sound very old-fashioned, then using the traditional forms would be appropriate; even if not, it's useful to know what they were. You might use "Miss" to address a female child under the age of thirteen.  Otherwise, if you don't know if they are married, then Ms. is a convenient get-out.
According to  The American Heritage Book of English Usage, using Ms. obviates the need for the guesswork involved in figuring out whether to address someone as Mrs. or Miss. Whether the woman you are addressing is married or unmarried, has changed her name or not, Ms. is always correct. In business correspondence, 'Ms' is almost always appropriate, but in social correspondence, one would do well to try to find out what the lady in question prefers. It is generally agreed that "Ms." is always appropriate unless the person in question specifies to the contrary.
Many non-feminist women don't like the title and object to having it applied to them, just as many feminist women object to the titles Miss and Mrs.
Many non-feminist women don't like the title and object to having it applied to them, just as many feminist women object to the titles Miss and Mrs.
Ms. applies to both married and unmarried women so you will never wrongly address a married woman as Miss or vice versa.  However, Ms. is politically charged to some extent because it has become connected with the feminist movement and some women do not wish to be associated with that movement - so call a woman by the title she prefers or use no title. Ms. Is convenient when you do not know how to address a women. Some women will be seriously offended if you don't address them by their preferred title, but in business correspondence Ms is almost always appropriate.
The title "Ms" started to be used in the 1970s by feminists who objected to being "identified based purely on their relationship with men". If you're writing historical fiction, or you want a character to sound very old-fashioned, then using the traditional forms would be appropriate; even if not, it's useful to know what they were. You might use "Miss" to address a female child under the age of thirteen.  Otherwise, if you don't know if they are married, then Ms. is a convenient get-out.
It is generally agreed that "Ms." is always appropriate unless the person in question specifies to the contrary.