initials vs monogram

initials

noun
  • An abbreviation of a person's name, constructed by taking the initial letters of first, last, and sometimes middle name(s). See monogram 

monogram

noun
  • A design or motif composed of one or more letters, often intertwined, used as an identifying mark of an individual or institution. 

verb
  • To mark something with a monogram. 

How often have the words initials and monogram occurred in a corpus of books? (source: Google Ngram Viewer )