digit vs letter

digit

noun
  • A distinct symbol representing one of an arithmetic progression of numbers between 0 and the radix. 

  • The whole numbers from 0 to 9 and the Arabic numerals representing them, which are combined to represent base-ten numbers. 

  • A narrow extremity of the human hand or foot: a finger, thumb, or toe. 

  • ¹⁄₁₂ the apparent diameter of the sun or moon, (chiefly) as a measure of the totality of an eclipse. 

  • A unit of length notionally based upon the width of an adult human finger, standardized differently in various places and times, (especially) the English digit of ¹⁄₁₆ foot, now equivalent to about 1.9 cm. 

  • Similar or similar-looking structures in other animals. 

verb
  • To point at or point out with the finger. 

letter

noun
  • A symbol in an alphabet. 

  • 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 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. 

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

  • To earn a varsity letter (award). 

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