flying buttress vs mainstay

flying buttress

noun
  • A buttress that stands apart from the structure that it supports, and is connected to it by an arch (flyer). 

mainstay

noun
  • A stabilising rope from the top of the mainmast to the bottom of the foremast. 

  • Someone or something that can be depended on to make a regular contribution. 

  • A chief support. 

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