Let be the horizontal line in the adjacent diagram. Angle (left of point ) is the subject of trisection. First, a point is drawn at an angle's ray, one unit apart from . A circle of radius is drawn. Then, the markedness of the ruler comes into play: one mark of the ruler is placed at and the other at . While keeping the ruler (but not the mark) touching , the ruler is slid and rotated until one mark is on the circle and the other is on the line . The mark on the circle is labeled and the mark on the line is labeled . This ensures that . A radius is drawn to make it obvious that line segments , , and all have equal length. Now, triangles and are isosceles, thus (by Fact 3 above) each has two equal angles.