monotone vs vain

monotone

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

  • Having a single unvaried pitch. 

noun
  • A piece of writing in one strain throughout. 

  • A single unvaried tone of speech or a sound. 

verb
  • To speak in a monotone. 

vain

adj
  • Having no real substance, value, or importance; empty; void; worthless; unsatisfying. 

  • Overly proud of oneself, especially concerning appearance; having a high opinion of one's own accomplishments with slight reason. 

  • Showy; ostentatious. 

  • Effecting no purpose; pointless, futile. 

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