In the passage from Emma in which Emma Woodhouse promises to call Mr. Knightley by his first name “in the building in which N. takes M.” for her wedded husband “for better, for worse”, Jane Austen is directly quoting from the “Form of Solemnization of Matrimony” (i.e. wedding ceremony) from the Church of England Book of Common Prayer.

In this prayer book, when a person’s name is to be said as part of a ceremony (such as baptism, etc.), then the place where the person’s name should be said is generally indicated by the letter “N.”, which stands for Latin nomen “name” (insofar as it stands for anything at all).

In the earlier versions of the prayerbook, this “N.” occurred wherever either the man’s name or the woman’s name was to be spoken as part of the wedding ceremony. In most later editions of the prayerbook, in order to prevent any possible confusion as to where the man’s name was to be spoken, and where the woman’s name, two different letters have been used — “M.” was introduced to mark places where the man’s name should be said, while “N.” was left to mark places where the woman’s name should be said. The differentiation was probably done in this way merely because the man’s name is usually said before the woman’s name in the ceremony, and “M.” comes before “N.” in the alphabet; as far as I’m aware, there is no deeper significance to the particular choice of letters for the man’s and woman’s names, and the letter “M.” doesn’t seem to abbreviate anything (in the way that “N.” can be said to abbreviate nomen). (One ingenious suggestion, that “M.” and “N.” were intended to stand for Latin maritus “husband” and nupta “bride”, must remain rather doubtful.)

“M.” and “N.” are never actually part of the ceremony as such (never spoken aloud), but are merely convenient little written markers to help tell the minister what he should say as part of the ceremony.