parapet vs picket fence

parapet

noun
  • A low protective wall. 

  • A fortification consisting of a wall. 

  • Part of a perimeter that extends above the roof. 

  • Hyponym: balustrade 

picket fence

noun
  • A simple fence made from wooden pickets, connected by horizontal bars. 

  • A series of spaced perpendicular striations that occasionally occur in aurorae. 

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