How a hole is drilled to be made square. The red shape in the center would be the cutting tool.
it shares the same principle as a Reuleaux triangle but with one rounded corner so that the cut square does not have rounded edges; the cutting tool follows the path of the rounded edge that is tangent to the sides of the outer square. because a tangent line is perpendicular to the radius, as the cutting tool follows the path of the rounded edge it turns precisely 90° to create a sharp edged perfect square.
(via visualizingmath)
