Angle Trisection -- from Wolfram MathWorld

Angle trisection is the division of an arbitrary angle into three equal angles. It was one of the three geometric problems of antiquity for which solutions using only compass and straightedge were sought. The problem was algebraically proved impossible by Wantzel (1836). Although trisection is not possible for a general angle using a Greek construction, there are some specific angles, such as pi/2 and pi radians (90 degrees and 180 degrees, respectively), which can be trisected. Furthermore

Maclaurin Trisectrix -- from Wolfram MathWorld

Trisecting the General Angle, A Plethora of Pretty Approaches - Pat'sBlog

Pat'sBlog: Trisecting the General Angle, A Plethora of Pretty Approaches

Mongean Projective Geometry Reverse Engineering

Angle Trisection Different Modes, PDF, Circle

Angle Trisection Different Modes, PDF, Circle

Angle - Wikipedia

Angle Trisection -- from Wolfram MathWorld

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Approximate Angle Trisection – GeoGebra