abbess vs prioress

abbess

noun
  • A female superior or governess of a nunnery, or convent of nuns, having the same authority over the nuns which the abbots have over the monks. 

prioress

noun
  • A nun in charge of a priory (usually lower in rank than an abbess); mother superior. 

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