What Does the M Mean on an RSVP Card?

The “M” on an RSVP card is a traditional etiquette prompt that stands for the first letter of your preferred title (such as Mr., Mrs., Ms., Miss, or Mx.).
It is simply a cue to let you know where to write your name(s) and how you would like to be addressed.Etiquette
How to Fill It Out
You simply complete the line by writing your title and name(s). For example:
Mr. and Mrs. John Smith
Ms. Jane Doe and Guest
Mx. Alex Taylor
Dr. and Mr. Sarah and David Jones
Why Do RSVP Cards Use an “M”?
Historically, formal wedding invitations and responses were written in the third person (e.g., “Mr. and Mrs. Smith request the pleasure of your company…”). The “M” was a formal, elegant way to prompt the guest to begin their response with their proper title, maintaining the formal tone of the invitation.
Modern Alternatives