a cappella vs plainsong

a cappella

noun
  • A vocal performance with no instrumental accompaniment. 

adj
  • Singing solely or mainly without instrumental accompaniment. 

  • Related to a form of purely vocal music mostly associated with American college performance groups. 

adv
  • In a manner of a choir with no instrumental accompaniment; literally, "in the style of the (Sistine) Chapel (in Rome)", such as a musical Mass done a cappella. 

plainsong

noun
  • A form of monophonic chant in unison using the Gregorian scale, sung in various Christian churches. 

  • A plain unvarnished statement, without exaggeration. 

  • A cantus firmus or theme chosen for contrapuntal treatment; so called because often an actual fragment of plain-song. 

  • The simple notes of an air, without ornament or variation. 

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