Addressing envelopes is an etiquette minefield. Although it is simple to address envelopes to a married couple with the same last name, less traditional living arrangements lead to more complicated addresses. If you are addressing an envelope to a mother and daughter, the rules vary depending on the daughter's age. A daughter over the age of 18 should receive her own mail, while a daughter under 18 should be addressed along with her mother.
Address only the mother on the outer envelope of formal mail if the daughter is under 18. On the inner envelope, write the mother's name and the daughter's first name. For example, on the outer envelope, you would simply write:
Mrs. Emily Davis
On the inner envelope, you would write:
Mrs. Davis (or "Emily" if the two of you are on a first-name basis) Shannon Davis
Address mail to a mother and minor daughter with no inner envelope by writing "Miss" or "Ms." followed by the daughter's name under the mother's name:
Mrs. Emily Davis Miss Shannon Davis
Address separate envelopes for the mother and daughter if the daughter is over 18 and you are sending a piece of mail such as a wedding invitation or graduation announcement.
Address the envelope as you would for a couple living together if you need to send a joint letter to a mother and her adult daughter. For example, if you are sending holiday cards, you may only want to send one card to each household. In that case, you would write:
Mrs. Emily Davis Ms. Shannon Davis