Monograms always seem to be confusing so I'm here to make it easy and simple!
•When the monogram is 3 letters and the center is larger it is ALWAYS the last name initial that is in the center. If it's a couple then the woman's initial is on the left and the man's initial is on the right. If the monogram is for a single person then the last name initial is center, the first initial is on the left and the middle or maiden name initial is on the right.
•When the monogram is a 3 letter monogram but all letters are the SAME size then it goes first, middle/maiden, last name initials.
•When using stationery and it's a single initial the rule of thumb is that if it's casual stationery you use the first name initial but if the stationery is formal you should use the last name initial. If you are using it for a couple then of course it would be the last name initial.
I hope this helps!