indistinguishable vs monotone

indistinguishable

adj
  • Not distinguishable; not capable of being perceived, known, or discriminated as separate and distinct 

  • Not capable of being perceived or known. 

noun
  • Any of a set of things that cannot be distinguished. 

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. 

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