@PinkMagnolia: Emily Post Says your way is the correct was to address correspondence. Mrs. Smith socially, Ms. Doe otherwise.
@JenGirl: Erm… no. Mrs. = Mistress.
Caveat: Usage and definition of honorifics change over time. Forgive my amature historian ways and the rambling, I studied romans and medieval monks, regency era is just a past-time.
18th-19th century (where the words are starting to come into the modern usage): A man was able to become “independent” of his family in several ways, this independence (a term used for the action of leaving the family and for the money/property/means that provided the ability to leave) would earn him the name “Master” that is, the ruler of his household (Household being the concept of family + tenants + clients, a much more complex definition than given to ‘household’ in the present day)
A gentleman (gentry) could 1) Come from a wealthy enough family to take over some of his father’s land for his living while his father was still alive (he would then be the steward or outright owner of these properties and act as a landlord), 2) or upon the death of his father he would take all of his father’s property as an inheritance. 3) Marry a wealthy woman (usually with a noble background/title to recommend him).
Merchants or non-gentry would usually just need to work as employees. I do not know for sure if they would have been called “Master” or not … probably mister or some variant.
A Mistress was the equivalent woman independent from her FATHER, who was the master of his household, she being under his care/rule. When she was able to run her own household, she was a mistress, Mrs. Though the dowry system is still in full force, the woman is not considered property as much as “under the care and protection of” her nearest male (or very wealthy spinster aunt) relative.
An adult female could achieve independence from her father by a 1) wealthy inheritance (which was almost always a stipend and not the deeds to the property that provided, these would go to the next living male heir) 2) Marrying a wealthy man (the age gap for marriage of men and women is important here,f: 18+ m: 23+, because a man would have schooling and some time to establish himself as independent from his father)
So… the most common method a woman was recognized as an adult female mistress of her own house in her own right, and not just her parents’ daughter, a girl was through marriage. Therefore, de-facto the unmarried woman is a Miss (a simple contraction of mistress) because it was assumed she was under the care of some relative who could provide for her.
Now we forget that in 19th century, later than the period above (word evolution!), Mister was used for adult males, and boys/young men were refered to as Master, but there are still some instances where formality dictates the use of ‘master’ for boys.