Pressure washers were first developed in the 1950s, and are now used primarily as exterior cleaning tools. Basic residential models consist of four parts: An electric or gasoline motor, a water pump, a high pressure hose and a sprayer, which is usually in "trigger gun" style - a long metal tube with a pressure-sensitive grip handle at one end. Most washers have connectors for a standard garden hose, which acts as their water supply. The internal pump on will provide a set amount of gallons per minute - which cannot be altered - but almost all include the ability to change pressure by adjusting the unloader valve on the sprayer.