The A19 light bulb is the most common residential light bulb. With light bulb names, letters are used to indicate light shape and numbers to indicate light size. The A19 has an arbitrary shape, with 19 being the widest width of the bulb in eighths of an inch.
In addition to bulb shape and size, selecting the correct base is important when choosing a light bulb. The two most common bases are the E and G. The Edison Screw, or E base, is named for Thomas Edison. It has a thread metal base that screws into a light socket. The G base is a bi-pin style of base.