In the Figure--> the driver A carries a pin or roller R that fits in the four radial slots in the follower B. Between the slots there are four concave surfaces that fit the surface S on the driver and serve to keep the follower from rotating when they are fully engaged. In the position shown, the pin is entering one of the slots, and, on further rotation of the driver, it will move into the slot and rotate the follower through 90°. After the pin leaves the slot, the driver will rotate through 270° while the follower dwells—i.e., stands still. The lowest practical number of slots in a Geneva mechanism is 3; more than 18 are seldom used. If one of the slot positions is uncut, the number of turns that the driver can make is limited. It is said that the Geneva mechanism was invented by a Swiss watchmaker to prevent the overwinding of watch springs. For this reason it is sometimes called a Geneva stop.