A ball valve, one type of quarter turn valve, is quite literally a ball placed in a passageway through which fluid flows. The ball has a hole through it, by which the valve opens and closes. When the ball is positioned so that the hole runs the same direction as the passageway, the fluid simply flows through the hole, and the valve is open. However, when the ball is positioned so that the hole is perpendicular to the passageway, the fluid cannot pass through, and the valve is closed. The ball is controlled from outside the valve, often with a handle that is turned 90 degrees, or a quarter turn, back and forth to open and close the valve.Because of the nature of the ball valve, it works very well for situations in which a flow needs to be completely shut off, such as the shut-off valve on the main pipeline.