As a rule of thumb, the outer envelope of your wedding invitation should be formal, with titles and full names, while the inner envelope is more informal, leaving out first names or titles and last names.
To a Married Couple:
On the outer envelope:
Mr. John and Mrs. Samantha Smith
Or
Mr. and Mrs. John Smith
On the inner envelope:
Mr. and Mrs. Smith
Or
John and Samantha
To a Married Couple That Use Different Last Names:
List the person you're closet with first on the outer and inner envelopes. If you're similarly acquainted with both, list them in alphabetical order.
On the outer envelope:
Mr. John Smith and Ms. Samantha Jones
On the inner envelope:
Mr. Smith and Ms. Jones
Or
John and Samantha
To an Unmarried Couple Living Together:
As with a married couple, both names should be included on the envelopes, but in this case, each name gets its own line.
On the outer envelope:
Mr. Joseph Smith
Ms. Rebecca Jones
On the inner envelope:
Mr. Smith
Ms. Jones
To a Same-Sex Couple:
Use the same rules as you would for any other unmarried or married couple. If the couple is married, list the names on the same line.
On the outer envelope:
Ms. Celine Elgin and Ms. Jacqueline Purcell
Or list their full names without titles:
Celine Elgin and Jacqueline Purcell
On the inner envelope:
Ms. Elgin and Ms. Purcell
Or
Celine and Jacqueline
To a Married Woman Doctor or Two Married Doctors
On the outer envelope:
If a woman uses her maiden name professionally and socially, the outer envelope should read:
Dr. Anne Barker and Mr. Peter Underwood
Or, if she uses her husband's name socially:
Dr. Anne and Mr. Peter Underwood
If both parties are doctors, you can address the outer envelope:
Doctors Anne and Peter Underwood
On the inner envelope:
Dr. Barker and Mr. Underwood
Or
The Doctors Underwood
To Those With Other Distinguished Titles:
Apply the same rules for military personnel, judges, reverends, etc., that you use for doctors. If both titles don't fit on one line, indent the second line.
On the envelope:
The Honorable Jane Kelly and Lieutenant Jonathan Kelly
Or
Captains Jane and Jonathan Kelly,
U.S. Navy
On the inner envelope:
Judge Kelly and Lieutenant Kelly, U.S. Navy
Or
The Captains Kelly
To Children and Families
Younger guests can be included on the inner envelope of their parents' invitation by their name(s) -- they should not be addressed on the outer envelope. For girls under 18, use "Miss". Boys don't need a title until they're 18 -- then they're addressed as "Mr."
Mr.and Mrs. Michael Abraham
Daniel, Jeffrey, Miss Brittany, and Miss Kelly
To Children 18 and Older
They should receive their own invitations (unless they live at home with mom and dad).
On the outer envelope:
Ms. Audrey Abraham
Or
Mr. Jack Abraham
On the inner envelope:
Ms. Abraham
Or
Mr. Abraham
Friendly note: If you don't include each child's name, you're implying that children are not invited. That said, don't be surprised if some guests still mistakenly assume their children are welcome. If you're concerned this will happen with your guests, ask your immediate family and bridal party to help spread the word that the wedding will be adults-only. In the end, you may have to follow up with guests who don't get the message via phone to gently explain the situation.
