Three letter monograms may be used for an individual or a couple.  

K = First name for an individual or the wife's name for a couple. (Ladies are always first.) 

P = Middle name for a single female, maiden name for a married female, or husband's name for a couple.

O = Surname in all three examples.

Two letter monograms may be used for an individual or a couple.  First and last name for an individual or two first names or last names for a couple.  In this example, the letters are overlaid and entwined, making it hard to tell which is the first letter, but if side by side letters, ladies are always first.

A single letter initial may be used to represent a first or last name.  Personal items tend to use first name initials, while home items use a last name initial more often.