What does the letter M mean on an RSVP card?

RSVP cards?

Traditionally, formal wedding invitations and responses to them were written in the third person (e.g. “Mr. and Mrs. Smith invite you to accompany you…”). The letter “M” was a formal, elegant way to encourage the guest to start their response with the right titlwhile maintaining the formal tone of the invitation. ShopParty Decor

Modern alternatives