gradual vs low-toned

gradual

adj
  • Proceeding or advancing by small, slow, regular steps or degrees 

noun
  • A service book containing the musical portions of the Mass. 

  • An antiphon or responsory after the epistle, in the Mass, which was sung on the steps, or while the deacon ascended the steps. 

low-toned

How often have the words gradual and low-toned occurred in a corpus of books? (source: Google Ngram Viewer )