reversal vs underside

reversal

adj
  • Intended to reverse; implying reversal. 

noun
  • A change in fortune; a change from being successful to having problems. 

  • An instance of reversing. 

  • The state of being reversed. 

underside

noun
  • The side that is below or underneath, the bottom. 

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