Traditionally wedding invitations have required two envelopes - an inner envelope that contains the actual invitations and inserts and an outer envelope to hold the inner envelope. While many brides continue to follow this rule, it is perfectly acceptable to omit the inner envelope.
Inner Envelope
The inner envelope only bears the names of the guests. No address is necessary. The inner envelopes should be addressed as follows:
Married Couple
Unmarried Woman and Guest
Miss Elizabeth Smith and Guest
Unmarried Gentleman and Guest
Mr. John Smith, Jr. and Guest
Engaged Couple
Miss Elizabeth Smith and Mr. John Doe
Unmarried Couple who lives together
Ms. Elizabeth Smith and Mr. John Doe
Several Young Children
Parents and a Young Child
Mr. & Mrs. John Smith
Miss Elizabeth Smith
Close Relatives
Aunt Elizabeth and Uncle John
Outer Envelope
No abbreviations are made on outer envelopes. Words naming roads such as Street and the names of states must be written out.
Wedding invitations are always addressed to both members of a married couple. Invitations to an unmarried couple should be addressed to Mr. John Doe and Ms. Elizabeth Smith.
Any child over the age of 13 should receive a separate invitation according to Emily Post. I'm not sure how practical that is. I think that any child over the age of 18 should receive a separate invitation and anyone younger can be included on the parents' invitation.
If you are allowing single guests to bring dates, the proper thing to do is to simply write your friend's name on the outer invitation and add the ...and Guest part to the inner envelope.
With so many unmarried couples living together nowadays, you may be confused as to how you should address the envelope. The proper thing to do is to address it to both of them even if you are really only friends with on of them.
Return Address
You need to include a return address on your wedding invitation. For one thing it allows the guest to know where to send the wedding gift. In addition, it lets the postal service know where to return it if it is undeliverable for some reason.
The return address is usually printed on the back flap of the envelope.
Stuffing the Envelopes
The invitation and additional inserts should be placed in the inner envelope with the invitation facing the back of the inner envelope so that it is the first thing the guest sees when opening the inner envelope. The front of the inner envelope should face the back of the outer envelope so that the guest will see their names on the front of the inner envelope when opening the outer envelope.
Works cited:
Post, Elizabeth L. Emily Post's Complete Book of Wedding Etiquette (Revised Edition). New York: Harper Collins Publisher, 1991.
