How to Address the Envelopes

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.