How Do You Address Invitations to Unmarried Couples?

Dear Ms. Cards Etiquette
 
My fiancé and I are getting ready to send out our wedding invitations. A lot of our friends are in relationships, but not married. What is the proper way to address an invitation to an unmarried couple?
 
The (Future) Robinsons

First off, congratulations on your (Future) wedding! Secondly, there are two ways to address a wedding
invitation to an unmarried couple, depending on whether or not they are living together:

Living Together

Outer Envelope

Write their names on separate lines, in alphabetical order:

Ms. Mary Ann
Mr. Tom Thumb
123 Address Street
City, ST 10000

Inner Envelope

Use the same convention, but using only their last names:

Ms. Ann
Mr. Thumb

Not living together

Outer Envelope

Send the invitation to the person you are closest friends with. For the outer envelope, address it to that person only:

Mr. Bob Knots
123 Address Street
City, ST 10000

Inner Envelope

Write both names, with your closest friend’s name first:

Mr. Knots
Ms. Black

Other Situations

The conventions for addressing the outer envelope in the examples above also apply to any other correspondence with an unmarried couple, in any situation where a formal address is appropriate.