What Is an E26 Lightbulb Socket? The Differences Between T2 & T3 Lighting Size of the Base Size of the Base The 12 portion of the base name refers to the size of the base. It's the diameter of the screw base in millimeters, so an E12 light bulb has an Edison Screw base that's 1...
which is another reason you might think they are interchangeable. Base size for A15 bulbs isn't universal, though; some have an E17 screw base, which is known as an intermediate base and is smaller than the E26 base (medium base) on a regular light bulb. Moreover...
also called an intermediate base. The E17 base is considerably wider and won't fit into a socket made for a candelabra bulb even though the actual bulbs may be of similar height and shape. If you aren't sure of the size of your burned-out bulb and therefore don't know...