To a Married Couple
Regardless if you are close with both people, you still need to include both names on the outer envelope. Your guests rely on the envelope to determine who is invited.
Mr. Bradley and Mrs. Rachel Gilmore
Mr. and Mrs. Bradley Gilmore
If you invitation includes an inner envelope, you can lose the first names or the titles and last names if you're close with the couple.
To a Married Couple With Different Last Names
List the person you're closest with first on the envelope. If you're well acquainted with both, list them in alphabetical order.
Mr. Roger Lamon and Mrs. Anna Thurton
To an Unmarried Couple Living Together or a Long Term but Unmarried Couple
Like a married couple, both names should be included on the envelope, but in this case, each name gets its own line.
Mr. Ryan Ferrer
Ms. Meghan Plight
If you're primarily friends with only one member of the couple, it's completely acceptable to address the envelope to that person and include "and Guest". If you are invited a single person with a guest write “and Guest” on the envelope. That being said some single guests may think they are invited with a guest even if it is not on the invitation. The best way to handle this is with a phone call or nicely worded email.
To a Same-Sex Couple
Use the same rules as you would for any other unmarried or married couple. If the couple is married or lives together, list them on the same line:
Married or Living Together
Mr. Foster Sheperd and Mr. Phillip Morgan
Married with same last name
Mr. Foster Sheperd and Mr. Phillip Sheperd
To a Married Doctor, Two Married Doctors or Other Distinguished Titles
If a woman uses her maiden name professionally and socially, the outer envelope should read:
Dr. Christina Young and Mr. Gregory Axter
If she uses her husband's name socially:
Dr. Christina and Mr. Gregory Axter
If both parties are doctors, you can address the outer envelope:
The Drs. Grammar
Drs. Maya and Ross Grammar
To a Married Male Doctor
Dr. and Mrs. Simon Richards
The same format is followed for other distinguished titles, such as Admiral, Reverend or Judge.
To Families with Children under 18
Younger guests can be included on the envelope of their parents' invitation by their name(s) or "and Family" -- although the latter is easier to confuse your guests.
Mr. and Mrs. Christopher Rhodes
Robert and Emily
Mr. and Mrs. Christopher Rhodes and Family
To Children 18 and Older
Children over 18 should receive their own invitations, although siblings over 18 can be sent a joint invitation. If this is the case the envelope should include their names alphabetically on individual lines.
Miss Emily Rhodes
Mr. Robert Rhodes
If you don't include "and Family" or each person’s name, you're implying that the children are not invited.