Book of Common Prayer

Book of Common Prayer (BCP) is the short title of a number of related prayer books used in the Anglican Communion, as well as by other Christian churches historically related to Anglicanism.

N or M?
The title of the 1941 novel by Agatha Christie, N or M?, is taken from a catechism in the Book of Common Prayer which asks, ''“What is your Christian name? Answer N. or M.”''. The "N. or M." here stands for the Latin, "nomen vel nomina", meaning "name or names". It is an accident of typography that "nomina" came to be represented by "m".