Is it addressed "and guest" or their name?

lindsaynewbride10

I've heard a lot recently that if the person you're inviting isn't engaged or married to their date, that you should address the invitation like Mr. John Smith and Guest. Is this a real etiquette rule?

LeahDVM2003

You only need to put the invitee's formal name on the outer envelope.  The inner envelope gets their name, in a casual format, and the "and guest".

Outer--Mr. John Smith
Inner--John and guest (If you normally call the invitee John, that's what you put on the inner envelope.  If you call him Mr. Smith, or Uncle John, or Grandpa, that's what you put.)

If they are a serious couple, or engaged, you should include the other person's name. 

Outer--Mr. John Smith and Miss/Ms. Jane Doe
Inner--John and Jane

 If they don't live together, you should send the SO his/her own invitation to his/her own address.