What is the etiquette for wedding invitation outer envelopes?

The outer envelope is what guests see first, so it sets the tone. Address it formally, using titles and full names. For a married couple, write 'Mr and Mrs John Smith' or, if they have different surnames, 'Mr John Smith and Mrs Jane Brown'. For unmarried partners, put each name on a separate line, with the person you know best first. For a single guest, use their title and full name, such as 'Ms Emily Jones'. If you are inviting a family, write the parents' names on the first line and 'and family' below, or list the children's names if you know them. Always include the full postal address, with the postcode on its own line. The outer envelope is sealed, and you should use the guest's preferred title if you know it. If you are unsure about someone's title, it is fine to use their full name without a title, or to ask a mutual friend. For same-sex couples, use the same rules as for any couple: both names, either on one line or two. The key is consistency and care. Handwriting the addresses looks personal, but printed labels are acceptable for a more casual wedding.