monotone vs monotonous

monotone

adj
  • Having a single unvaried pitch. 

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

verb
  • To speak in a monotone. 

noun
  • A piece of writing in one strain throughout. 

  • A single unvaried tone of speech or a sound. 

monotonous

adj
  • Having an unvarying pitch or tone. 

  • Tedious, repetitious, or lacking in variety. 

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