impact vs letter

impact

verb
  • To stamp or impress onto something. 

  • To collide or strike, the act of impinging. 

  • To significantly or strongly influence or affect; to have an impact on. 

  • To compress; to compact; to press into something or pack together. 

noun
  • A significant or strong influence or effect. 

  • A forced impinging. 

  • The striking of one body against another; collision. 

  • The force or energy of a collision of two objects. 

letter

verb
  • To print, inscribe, or paint letters on something. 

  • To earn a varsity letter (award). 

noun
  • A size of paper, 215 mm × 280 mm. 

  • Literature. 

  • A size of paper, 8½ in × 11 in (215.9 mm × 279.4 mm, US paper sizes rounded to the nearest 5 mm). 

  • A symbol in an alphabet. 

  • A division unit of a piece of law marked by a letter of the alphabet. 

  • One who lets, or lets out. 

  • The literal meaning of something, as distinguished from its intended and remoter meaning (the spirit). 

  • A written or printed communication, generally longer and more formal than a note. 

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