freeboard vs parapet

freeboard

noun
  • The distance between a water level and the top of something that contains or restrains it (such as a dam). 

  • The distance between the top of sea ice and the water level. 

  • A type of skateboard which simulates the movement of a snowboard when used on a downhill coarse, allowing snowboarding techniques, which has an addition of two centerline casters that extend below the traditional skateboard wheels and bogies. 

  • The vertical distance between the waterline and the uppermost watertight deck of a vessel. 

parapet

noun
  • Part of a perimeter that extends above the roof. 

  • A fortification consisting of a wall. 

  • A low protective wall. 

  • Hyponym: balustrade 

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