monotone vs off-key

monotone

adj
  • Having a single unvaried pitch. 

  • Being, or having the salient properties of, a monotone function. 

noun
  • A piece of writing in one strain throughout. 

  • A single unvaried tone of speech or a sound. 

verb
  • To speak in a monotone. 

off-key

adj
  • Pitched inaccurately either above or below the correct note. 

  • Not harmonious, or out of accord. 

adv
  • With inaccurate pitch. 

How often have the words monotone and off-key occurred in a corpus of books? (source: Google Ngram Viewer )