obverse vs tails

obverse

noun
  • The heads side of a coin, or the side of a medal or badge that has the principal design. 

  • A proposition obtained by obversion, e.g. All men are mortal => No man is immortal. 

adj
  • Corresponding; complementary. 

  • Having the base, or end next to the attachment, narrower than the top. 

  • Turned or facing toward the observer. 

tails

noun
  • The side of a coin that doesn't bear the picture of the head of state or similar. 

  • Tailings; waste. 

  • Tailings, feints. 

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