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The term A19 is used to describe the overall shape and dimensions of a light bulb. It has been used since the era of incandescent light bulbs, and now CFL and LED light bulbs continue to use the same term. An A19 bulb, therefore, has a diameter of 19 divided by 8 inches, or approximately 2.4 inches.
Correspondingly, are e26 and a19 the same?
To answer the question about whether A19 and E26 bulbs can be the same or not, YES they can. In the US, A19 bulbs must always have E26 type bases. This is based on ANSI standards imposed for bulb manufacturers in the US. That said, all A19 bulbs have E26 bases so all A19 bulbs are the same as E26 bulbs.
Subsequently, question is, what is the standard light bulb base?
E26 is the size of most light bulbs used in the U.S. It's referred to as having a “medium” or “standard” base. E12 is the smaller “candelabra” base.
A19 refers to the shape and size of the bulb itself. "A" stands for Arbitrary -- just the name of the shape that's familiar to most of us. "19" refers to the size -- in this case, 19/8" (or 2 3/8") across. E26 refers to the type and size of the base -- the part that goes into the light socket.