Screws are an example of a basic machine. They have a thread, which is a corkscrew-shaped ridge wrapped around a cylinder. The head is built specifically to allow a screwdriver or wrench to hold the screw while driving it in. There are three types of screws used in household applications: wood, metal and plastic. Screws come in many sizes for various projects. The most common sizes are 1/4", 3/8" and 1/2". These numbers indicate the width of the screw's thread in millimeters. So, a 3/8" screw has a thread that is 0.040" wide. Some larger screws can be found with 5/8" or 7/8" threads; these are usually used in construction projects as they can be driven into concrete or steel without stripping the thread. Sometimes, especially with metal screws, there will be different colors used to identify their purpose-for example, black for electrical connections or white for plumbing work. Screws are used everywhere in the home improvements market today. They're used to attach cabinets to walls, windows to doors, and even light fixtures to ceilings. Sometimes several pieces of equipment are attached using separate screws. For example, if a cabinet has two sides and two backs, then eight screws would be needed to attach it to the wall.