can slide over the impulse plane of the exit pallet, which transfers a clockwise impulse to the balance wheel's impulse pin via the lever fork, while pushing the lever up against the left banking pin. The escape wheel drops again until the entrance tooth locks on the entrance pallet now being held in place by the left banking pin via the lever. The balance wheel continues clockwise, again free from interference until it is pulled back by the hairspring to the center position. The cycle then starts again. Each back and forth movement of the balance wheel from and back to its center position corresponds to a drop of one tooth (called a beat). A typical watch lever escapement beats at 18,000 or more beats per hour. Each beat gives the balance wheel an impulse, so there are two impulses per cycle. Despite being locked at rest most of the time, the escape wheel rotates typically at an average of 10 rpm or more. The origin of the "tick tock" sound is caused by this escapement mechanism. As the balance wheel rocks back and forth, the ticking sound is heard. Draw The reliability of the modern lever escapement depends upon draw; the pallets are angled so that the escape wheel must recoil a small amount during the unlocking. The draw holds the lever against the banking pins during the detached portion of the operating cycle. Draw angle is typically about 11-15 degrees to the radial. Early lever escapements lacked draw (indeed some makers considered it injurious as a cause of extra friction in unlocking); as a result a jolt could result in the escapement unlocking. Lever watch movement Most modern mechanical watches are jeweled lever watches, using synthetic ruby or sapphire jewels for the high-wear areas of the watch. Pin pallet escapement A cheaper, less accurate version of the lever escapement is used in alarm clocks, kitchen timers, mantel clocks and, until the late 1970s, cheap watches, called the Roskopf, pin-lever, or pin-pallet escapement after Georges Frederic Roskopf, who invented it in 1867. It functions similarly to the lever, except that the lever pallet jewels are replaced by vertical metal pins. In a lever escapement, the pallets have two angled faces, the locking face and the impulse face, which must be carefully adjusted to the correct angles. In the pin pallet escapement, these two faces are designed into the shape