The Tusi couple is a 2-cusped hypocycloid obtained by rolling a circle of radius http://mathworld.wolfram.com/images/...le/inline1.gif inside a circle of radius http://mathworld.wolfram.com/images/...le/inline2.gif. The result is a line segment (Steinhaus 1999, p. 145; Kanas 2003), as noted by the Persian astronomer and mathematician Nasir Al-Din al-Tusi (1201-1274) (Sotiroudis and Paschos 1999, p. 60; Kanas 2003).