The letter “M” printed on many formal RSVP cards often confuses people who are unfamiliar with older invitation customs. Its purpose is simple. The “M” is the beginning of a courtesy title that the guest is expected to complete before writing their full name. Traditionally, guests continued the letter by adding “r.” for Mr., “rs.” for Mrs., “s.” for Ms., or “iss” for Miss, producing lines such as “Mr. John Smith” or “Ms. Jane Smith.”

This format comes from older standards of formal etiquette when using courtesy titles was expected. Including only the first letter allowed each guest to choose the title that suited them while keeping the card looking elegant and uniform. Modern invitations have become far more flexible. Guests who do not use traditional titles are free to write the form of address they prefer or simply write their name, depending on the space provided. The main goal of the line is to identify who is responding, not to force anyone into a particular style.