The term defines the property of a curve placed on a vertical plane for which a body, starting from a position of rest and traveling down the curve with natural motion, always takes the same time to reach the lowest point, regardless of the point on the curve from which it begins to move. Galileo (1564-1642) was the first to assert the tautochronism of motions along the quarter-circle. Christiaan Huygens (1629-1695) later demonstrated that the only truly tautochronous curve is the cycloid.