antiphony vs introit

antiphony

noun
  • alternate, or responsive singing by a choir split into two parts; a piece sung or chanted in this manner 

  • alternate, or responsive ideas or opinions; juxtaposition 

introit

noun
  • A composition of vocal music sung at the opening of a church service. 

  • A prayer, typically part of a psalm or other portion of the Bible, read or sung at the start of Mass while or immediately after the priest ascends to the altar. 

  • An anthem or psalm sung before a Communion service 

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