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From Citizendium, the Citizens' Compendium

Euclidean geometry is a form of geometry first codified by Euclid in his series of thirteen books, The Elements.

Concepts

Some of the concepts used and described in Euclidean geometry are:

• point
• line or straight line
• surface
• plane
• plane angle
• rectilinear angle or angle including
  • right angle
  • obtuse angle
  • acute angle
  • straight angle
• parallel
• perpendicular
• boundary
• figure
• plane figure
  • rectilinear figure
  • triangle
    • equilateral triangle
    • isoscelese triangle
    • scalene triangle
    • right angled triangle
    • obtuse angled triangle
    • acute angle triangle
  • polygon
    • square
    • oblong
    • rhombus
    • rhomboid
    • trapezia
• circle
• center
• diameter
• semicircle
• trilateral
• quadrilateral
• multilateral
• equilateral