A socket wrench is a tool used to grip the head of a fastener such as a nut or bolt to tighten or loosen it. It comes with a long handle to which different size sockets can be attached for tightening different size fasteners. Sometimes the socket wrench has a ratchet handle, allowing the wrench to turn continuously without having to lift up the tool. For each particular job, it's important to choose the size socket wrench that corresponds with the particular hardware.

There are four common socket sizes: 1/4 inch (0.6 centimeters), 3/8 inch (0.9 centimeters), 1/2 inch (1.3 centimeters) and 3/4 inch (1.9 centimeters). In addition, sockets are available in various shapes to fit the different bolt heads: six points (hexagon), eight points (double square), and 12 points (double hexagon). If you choose a socket that's too large, you could damage the tool and injure your hands. Furthermore, some socket wrenches are deeper than normal to allow for bolts that protrude.

Before embarking on a project, for instance replacing your car's engine oil, you need to decide which size socket wrench to use. You need a socket wrench to open the oil drain plug so the used oil can drain from the engine. First, turn on the car to warm up the engine. This thins the oil, allowing it to drain smoothly. Then turn off the engine, firmly set the parking brake and place jack stands under the car. Next, locate the drain plug under the car; it should still be warm from the heated engine. Place a 3/8-inch (0.9 centimeters) socket in the socket wrench, fit it over the drain plug's nut and turn it counterclockwise. The oil drain plug should open easily, draining the used oil from the engine. Make sure to place a bucket and some newspaper under the car's engine to catch all the draining oil.