High Quality Content by WIKIPEDIA articles! The Tusi-couple is a mathematical device in which a small circle rotates inside a larger circle twice the radius of the smaller circle. Rotations of the circles cause a point on the circumference of the smaller circle to oscillate back and forth in linear motion along a diameter of the larger circle. It was developed by the 13th-century Persian astronomer and mathematician Nasir Al-Din Tusi as an alternative to the problematic equant introduced over a thousand years earlier in Ptolemy's Almagest. Some modern commentators also call the Tusi couple a "rolling device" and describe it as a small circle rolling inside a large fixed circle.