The use of double envelopes (outer & inner) dates back to the time when invitations were delivered by hand. Back then envelopes would be left unsealed for courtesy reasons. With the invention of the postal service, the unsealed inner envelopes would be placed into larger sealable envelopes. Today, some couples choose to keep the tradition of the inner envelope.
There are different ways in which you could address an inner envelope. The formal approach would be to use titles and no first name (ie. Mr. & Mrs. Smith). Another approach is to think about what you would call your guest if you called them on the phone… Dad or Dr. Jones or Sam?
Calligraphy in Olivia: