Hello! What is the proper way to address wedding invitations?

Live with a significant other:

Mr. Adam West and Ms. Vicki Vale -outside envelope

Adam and Vicki -- inside envelope

Large family of living together couple:

If you want to invite all the children, find out their names if you can.

Outside envelope: Mr. Adam West and Ms. Vicki Vale and children

Inside envelope: Adam and Vicki

Charlie, Bill, Fred, George, Ron and Ginny (without regard to which are Wests and which are Vales)

though you could write:

Charlie and Ginny West

Bill, Fred, George and Ron Vale

Guest with significant other not living together:

Mr. Harry Potter -- outside envelope

Mr. Harry Potter and Ms. Hermione Granger -- inside envelope.

If you know, say, the wife but not the husband, do find out his name, and address it the same as if you know both members of a married couple.

Mr. and Mrs. James Potter -- outside envelope

James and Lily -- inside envelope

Same Sex Couples -- most of the time they don't take one name even when they marry, so it's the same as a living together couple.

Mrs. Ellen Degeneres and Mrs. Portia DeRossi -- outside envelope

Ellen and Portia -- inside envelope

If they are married and took one name:

Messrs Adam and Anthony West -- outside envelope

Adam and Tony - inside envelope

For long term, living together, engaged or married couples, you put both of their names.

Mr. John Smith and Ms. Jane Doe

Mr and Mrs. John Smith.

For short term couples, you put the name of the guest that is connected to you and then "and guest". This covers potential break ups of short term couples. If you don't want them to bring some random person, if they do happen to break up between invitation time and the wedding, you can always treat these invites as you would above and put both names down.

Mr. John Smith and Guest.

Ms. Jane Doe and Guest.

For large families, you can either put the name of the guest connected to you and "and Family" or you can simply put "The Johnson Family." If you're having an inner and outer envelope, you can list the names of those invited on the inner envelope, although this isn't necessary.