flagstone vs mattress

flagstone

noun
  • A flat, rectangular piece of rock or stone used for paving or roofing. 

  • One of several types of rock easily split and suitable for making flagstones. 

mattress

noun
  • A form of retaining wall used to support foundations or an embankment 

  • A pad on which a person can recline and sleep, usually having an inner section of coiled springs covered with foam or other cushioning material then enclosed with cloth fabric. 

verb
  • To cover with a thick layer, like a mattress; to blanket. 

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