The instrument known as "Jacob's staff," attributed to Jacob ben Machir ibn Tibbon (1236-1304), was adopted in astronomy under the name of "astronomer's staff" and in navigation under the name of "cross-staff." Its operating principle was similar to that of the Arab kamal, introduced in Portugal by Vasco da Gama (1448-1524) as tábuas-da-índia. To determine the altitude of a celestial body above the horizon with this type of instrument, the user would bite one end of a string carrying knots and stretch it out in front of him. The string was attached at the far end to a wooden rod, which, depending on the distance from the user's eye, subtended angles of varying widths. Probably introduced in Spain by the Turks, the cross-staff performed the same function, replacing the string with a graduated rule or staff. One of three or four interchangeable vanes of different lengths could be mounted cross-wise on the rule. The observer would slide the vane toward or away from his eye, until it subtended the angle between the celestial body and the horizon.