bit part vs principal part

bit part

noun
  • A minor role, generally not assigned a name. 

principal part

noun
  • A polynomial approximation of a power series, made up of monomials whose indices lie in the Newton diagram of the power series and which occur with the same coefficients as in the original power series. 

  • The portion of a Laurent series that has negative exponents. 

  • Any of the forms of a word which contain its stem(s) in the simplest form, or such a form that, when taken with all the other principal parts, allows the entire paradigm to be derived. 

  • Used other than figuratively or idiomatically: see principal, part. 

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