The A-series light bulb is the "classic" type of light bulb that has been the most commonly used type for general-purpose lighting applications since the early 20th century. It has a pear-like shape and an Edison screw base. The number that follows the "A" within the A series indicates the width of the bulb in one-eighth inch units or in millimeters.[1][2][3]
The most commonly used A-series light bulb type is the A19 bulb (or its metric equivalent, the A60 bulb[2]), which is 2 3⁄8 inches (60 mm) wide at its widest point[3] and approximately 4 3⁄8 inches (110 mm) in length, and has a one-inch long (type E26, i.e. 26 millimeters in diameter) Edison screw base. Other base types most commonly used in UK, Australia, and New Zealand include the E27 (i.e. 27 millimeters in diameter) Edison screw base (which also fit E26 sockets), and less commonly these days;[citation needed] the B22 Bayonet twist type base.