page_title: Enneadecagon
section_title: Construction
hierarchical_section_title: Enneadecagon / Regular form / Construction
context_page_description: In geometry an enneadecagon or enneakaidecagon or 19-gon is a nineteen-sided polygon.
context_section_description: As 19 is a Pierpont prime but not a Fermat prime, the regular enneadecagon cannot be constructed using a compass and straightedge. However, it is constructible using neusis, or an angle trisector.
  Another animation of an approximate construction.
  Based on the unit circle r = 1 [unit of length]
  Constructed side length of the enneadecagon in GeoGebra  [unit of length]
  Side length of the enneadecagon [unit of length]
  Absolute error of the constructed side length [unit of length]
  Constructed central angle of the enneadecagon in GeoGebra 
  Central angle of the enneadecagon 
  Absolute error of the constructed central angle 
  Example to illustrate the error
  At a radius r = 1 billion km (a distance which would take light approximately 55 minutes to travel) the absolute error of the side length constructed would be approx. 0.21 mm.
caption: Regular enneadecagon, exact construction using the quadratrix according of Hippias as an additional aid