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Bulb Shape 

Every bulb is identified by a unique letter and number combination known as the Bulb Designation. A Bulb Designation consists of a letter(s) to indicate the shape and a number(s) to indicate the approximate maximum diameter in eighths of an inch. So, an ED17 is an Ellipsoidal Shape, 17/8 of an inch or 2 1/8 inches in diameter bulb.  A bulb can also be identified by its Base type.