introit vs psalmody

introit

noun
  • An anthem or psalm sung before a Communion 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. 

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

psalmody

noun
  • The singing or the writing of psalms. 

  • A collection of psalms. 

verb
  • To celebrate in psalms. 

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