freeboard vs linchpin

freeboard

noun
  • 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 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. 

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

linchpin

noun
  • A pin inserted through holes at the end of an axle or shaft, so as to secure a wheel or shaft-mounted device. 

  • A central cohesive source of stability and security; a person or thing that is critical to a system or organisation. 

verb
  • To adopt as, or serve as, a central cohesive source of stability and security. 

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