The earliest turret-clock escapements were of the verge-and-foliot type. There was the occasional variant like the one used by Richard of Wallingford, comprising two wheels on the same axis between which the balance was pivoted. A curved bar at the bottom of the balance engaged alternately the spokes of the two wheels. After Christiaan Huygens developed the pendulum in 1650 a number of clocks were built with crown wheel and large-arc bob. The improved timekeeping of the recoil anchor escapement in 1671 brought a demand for conversion from verge-and-foliot, and the dead-beat escapement also found wide application in turret clocks for many years. It appears in various forms apart from the normal anchor type. In some the pallets are adjustable, either by sliding through the bow of the anchor or by a two-part bow, the relative positions of which can be adjusted by a screw. In one variant the teeth protrude axially from a disc rather than radially from the wheel's periphery. The pinwheel was also widely used. Evidently it had a good reputation, as a few recoil escapements were converted to pinwheel by the expedient of inserting pins in the rim of the wheel, leaving the teeth in position. Some very large pinwheels were made, and sometimes the pins fell on pallets fixed directly to the pendulum and not through a crutch. A major improvement was the gravity escapement. The commonest version is the double three-legged. As the pendulum swings, it engages the beat pin on one of the gravity arms, moving it outwards. This unlocks the arbor which rotates 1/6 turn, locking on the other gravity arm. During the 1/6 turn, one of the three lifting pins has lifted the gravity arm a little as the pendulum moves. On the return stroke, therefore, the gravity arm moves under its own weight a greater distance than it moved out. The difference of the two movements represents the energy delivered to the pendulum, i.e. The impulse. The importance of the gravity escapement is that the impulse delivered to the pendulum is constant and quite independent of the load on the clock resulting from wind and weather. The fly on the 'scape arbor is essential to provide enough damping to prevent tripping. Other versions of this escapement have appeared - double four-legged and single three-legged. In one version, the impulse takes place on one side only. Commonly 15 legs are used.