Addressing Etiquette

Ever been curious how to address your envelopes for your big day? Ever know how to address a guest that is living with someone but unmarried? Did you even know there was a particular "etiquette" involved in the addressing? Well don't worry, I'm here to save any concerns you may have.
Follow these steps, and you'll be sure to not insult anyone and definitely impress!
(Feel free to use these for any other formal event you may have, too.)

Envelopes: 

Inner Envelope: includes the title and last name of the specific people invited. This allows the host to be very clear about who is invited, and by omission, who is not invited. If both adults and children are invited, list the couple's name and below it, the name of the child (separate lines).

Outer Envelope:
Names: Addressed conventionally using titles, first and last names. Titles and suffixes can be abbreviated (i.e. Mr., Mrs., Ms., etc.)
Addresses: Words such as "Street" or "Boulevard" should be spelled out! State names may be written in full or use the two-letter postal code abbreviation.

RSVP Envelope:
Write out all words here, too. House numbers and zip codes should be written in numeral form except for "one," which is spelled out. The return address is typically printed on the outer envelope's back flap.

Guests:

Married couple: Invitations are allowed addressed to both members of a married couple, even though the bride may know only one or knows that only one will attend.
Informal: You can include the first names of both husband and wife after their titles. The inner envelope can address the couple by their first names.
Different last names: In all cases, the wife's name is traditionally written first. This does not change if they have different last names!

Un-married Couple: Connecting the couple's names by the word "and" implies marriage. For an un-married couple that lives together, names should be written on separate lines without the word "and." On the inner envelope, both are addressed by their titles and respective last names.

Family with children:
Formal: The outer envelope would be the same as a couple without children. The inner envelope can specify the children by name that are invited. Boys under age 13 are addressed as Master and girls under age 18 are addressed as Miss.
Informal: The outer envelope would have both parents names written. The inner envelope can state the parents' and children's first names without titles and using the word "and" is acceptable to link the children's names.

Single Guest: For a single woman, either "Ms." or "Miss" is appropriate. The guest's name is the only one that appears on the outer envelope. The inner envelope can include the guest's name followed by "and Guest." If you know who their guest would be, it's more personal to include that person's name on a separate line.