countess vs peeress

countess

noun
  • The wife of a count or earl. 

  • A woman holding the rank of count or earl in her own right; a female holder of an earldom. 

peeress

noun
  • A noblewoman married to a peer. 

  • A woman holding a noble title in her own right. 

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