There are a number of etiquette rules to keep in mind when addressing wedding invitations so you don't offend a guest. You want to be sure to list each person invited to the wedding on the invitation. For example, if a family is invited, this would mean writing on the inner envelope the names of the children, oldest first. You would also want to include an "and guest" addendum on the inner envelope. This helps keep your guest list intact while graciously telling guests exactly who is invited.
Address single men, regardless of whether they were previously married, as "Mr." For example, "Mr. Eric Won."
Address single, unmarried, female guests as "Miss" or "Ms." A divorced female guest can use either "Mrs." or "Ms." if she kept her married name, but if she uses her maiden name, use "Ms." Finally, for a widowed female use "Mrs." Be careful with a widowed female -- don't include "and guest" on the inner envelope if she is recently widowed. The same applies to a recently widowed male guest.
Write children's names on the inner envelope if they are under 18 years of age. If the child is over 18, send an individual invitation addressed to "Miss" or "Mr."
Write the names of unmarried couples with the female's name first. If the couple does not live together, only address the member who lives at the address and put both names on the inner envelope.
Address married couples as "Mr. and Mrs." followed by the husband's full name. For example, "Mr. and Mrs. Matt Donovan." If the woman kept her maiden name or has a hyphenated last name, list each member of the couple individually with the woman's name first.
Address married couples who have professional titles by the professional title first. For example, "Doctor Elizabeth Bennett and Mr. John Bennett." Always spell out the word "doctor." If both husband and wife are doctors, write down each name, wife first, or write "the doctors." For example, "The Doctors Bennett."
Write same sex couples in alphabetical order by last name. If the couple has had a commitment ceremony, the inner envelope should be addressed to the couple, such as "The Gilberts."
Address invitations for military couples with the commissioned officer first, followed by the wife's married name. For example, you would write "Colonel and Mrs. Riley Davis." If he is a non-commissioned officer, use the rules for a married couple. If the wife is a commissioned officer, list each name individually, with her name first. In this case, "Captain Sara Davis and Mr. Riley Davis."
Address a judge as "The Honorable" and a reverend as "Reverend." Follow the same rules as a military couple for the spouse's name: "The Honorable and Mrs. Riley Davis" or "Reverend and Mrs. Riley Davis."