Using Miss & Ms.

Ms. was initially used when speaking to a woman and the speaker was unsure of the status of the woman being addressed.  We don't want to offend the person being addressed, so often we'll "slur" the Mrs. to Ms.  Thus Ms. was born!!  TA! DAH!!  With time, it has developed a place of it's own.  

When addressing invitations, I use Miss for the young, unmarried ladies.  However, when they are over age 30, I tend to use Ms.  As I mentioned in an earlier post, I have found that many divorced women prefer it to the Mrs.  Also, while researching the topic, I did find that some people use Ms. for young, unmarried ladies over 18.  For me, I think I'll stick to 30!!