Trisecting an angle (Abe’s method)

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1. 

   1. Two evenly spaced creases

   White side up. Fold the bottom edge to the top edge and unfold; then fold the bottom edge to that middle crease and unfold.

2. 

   2. Make a 60° angle

   Fold the top-left corner down onto the middle crease, with the crease starting at the bottom-left corner, then unfold. The crease makes 60° with the bottom edge: this is the angle we will trisect.

3. 

   3. Two points onto two lines (Abe’s fold)

   Fold the lower-left part over so that the bottom-left corner lands on the quarter crease and, at the same time, the left end of the middle crease lands on the 60° crease.

4. 

   4. Crease along the folded quarter line

   Crease through all layers along the quarter crease on the folded flap, then unfold just this crease (keep Abe’s fold). Extended, this line runs exactly through the tip of the angle, where the corner used to be.

5. 

   5. Unfold and extend the crease

   Unfold Abe’s fold. Fold along the new crease again, extending it all the way to the bottom-left corner, and unfold. This is the first trisecting line (40°).

6. 

   6. Bottom edge onto the 40° crease (20°)

   Fold the bottom edge up to lie exactly on the 40° crease, creasing from the bottom-left corner. This fold is the second trisecting line (20°).

7. 

   7. Fold once more to check

   Fold all layers up along the 40° crease. The folded edge lands exactly on the 60° crease: the 60° angle is split into three equal 20° parts!