Etiquette rules on addressing envelopes used to be more complex. Today, those rules are more relaxed and most couples with different last names are treated the same, regardless of their relationship.

When the last names of a husband and wife differ, both names should be written out on a single line. The woman’s name usually goes first. In this situation, you would address the envelope to “Ms. Jane Doe and Mr. John Jones.” Older etiquette rules required the names of unmarried couples to be placed on separate lines on the envelope without a conjunction joining them. Today, the rules have relaxed and their names can be treated the same way as a married couple who has different last names. Etiquette experts disagree on whether the man’s or the woman’s name should be written first. To avoid offending your recipients, write the names in alphabetical order. Address envelopes to same-sex couples who share a home the way you would to a married couple with different last names or an unmarried couple living together.

While sending one invitation per household may be tempting, roommates should receive separate invitations. If budget concerns override etiquette guidelines, place each roommate’s name on a separate line. Arrange the names in alphabetical order by last name.

If you are writing to adult siblings who live together, they probably share the same last name and can be addressed as “Misses Jane and Jennifer Doe” or “Messrs. John and James Doe.” When the last names do differ, write each name on a separate line on the envelope. For party etiquette, send each sibling a separate invitation.