A formal invitation will have an unsealed inner envelope slipped inside of the outer envelope. Even if you are hiring a calligrapher to do your invitations for you, it is nice to know the proper procedure:

Single Person
Mr. Michael Smith
64 Maple Street
Denver, CO 01234
(Send a separate invitation to guests such as dates or roommates.)

Married Couple
Mr. & Mrs. Michael Smith
64 Maple Street
Denver, CO 01234

Unmarried Couple living together
Mr. Michael Smith
Ms. Michelle Warner
64 Maple Street
Denver, CO 01234

Children
Children who are under the age of 18 do not need to appear on the invitations. If the child is over 18, you can include them on their parents' invitation or send their own.
Mr. & Mrs. Michael Smith
Rachel Smith
64 Maple Street
Denver, CO 01234

Single Person
Mr. Smith

Married Couple
Mr. & Mrs. Smith

Unmarried Couple living together
Mr. Michael Smith and Ms. Michelle Warner

Children
Mr. & Mrs. Smith
Rachel, Christopher, and Kyle

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