The Tusi couple is a 2-cusped hypocycloid obtained by rolling a circle of radius 
inside a circle of radius 
. 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).
