The A21 term for light bulbs is used to describe the overall shape and dimensions of a bulb. The A21 bulbs are larger than A19 bulbs and are more commonly used for higher light output applications.  The letters designate the bulb shape while the number represents the diameter in eight of an inch. For A21 the "A" refers to the overall shape of the bulb that resents a classic light bulb shape, commonly compared to an upside-down pear.  The 21 represents the bulb diameter in eight of an inch, more exactly 21/8 = 2.625 inch (or 2 5/8 inch).  The length of an A21 bulb is typically 4.1 - 5.4 inches (105-140 mm) in length, slightly longer than A19 bulbs.