How should I address people on my invitations?

If old-school etiquette is your thing, here are the "rules":

- Entire names should be written on the outer envelopes

- Married couples: "Mr. and Mrs." followed by the husband's first and last name or just list both full names.

- Unmarried couples: write the names on two lines.

- Kids: you can list children (under 18) just on the inner envelope of the invitation (or include them on the outer envelope too). You usually just list first names under the parents' names: 

Mr. and Mrs. Smith 
Peter, Paul, and Mary

"Kids" over 18 generally get their own invitation. 

-Include any appropriate titles (e.g. "Doctor")