A-series light bulb

It has a pear-like shape and is typically fitted to either an Edison screw or a bayonet cap base.

The number that follows the "A" designation indicates the nominal major diameter of the bulb, either in one-eighth inch units in North America[1][2][3] or in millimeters in the rest of the world.

In countries with a mains supply voltage of 100–120 volts (e.g., Canada, Taiwan, US), A19/A60 light bulbs usually come with an E26 type Edison screw base (i.e. 26 millimeters in diameter).

In the UK, Ireland and many Commonwealth countries, they usually come with a B22 type bayonet cap base.

ANSI C79.1-2002, IS 14897:2000,[5] and JIS C 7710:1988[9] define the "A shape" as "a bulb shape having a spherical end section that is joined to the neck by a radius", where the radius is greater than that of the sphere, corresponds to an osculating circle outside the light bulb, and is tangent to both the neck and the sphere.