plainsong vs sacred song

plainsong

noun
  • 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. 

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

sacred song

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