Traditional monogram letters are the first, middle and last initials, with the last initial centered and in a larger font. A married woman may use her first initial and the initials of her married and maiden names instead.
It is also acceptable to put first, middle and last initials in order in the same size font. This second monogram style is more often used for a man or a single woman. Children and married women usually use the first, last and middle monogram form.
Two other less-used monogram forms use only the last name. The first form is simply the last initial. The second form allows for hyphenated last names or cases in which the wife chose to keep her maiden name. It uses both last-name initials with a pipe character between them: "X | X."