heretofore vs previously

heretofore

adv
  • Before now, until now, up to the present time; from the beginning to this point. 

  • Previously within the present text; earlier herein. 

previously

adv
  • At an earlier time. 

  • Recapitulating the preceding episodes of a series. 

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