E27, E14, GU10, GU5.3, G9, G4, GX53… these base codes look like a code, and a mistake means another trip to the store—or a light bulb that stays in the drawer forever. In fact, the system is logical and can be figured out once and for all. In this guide, we explain what the letters and numbers in bulb base names mean, how to identify the base in your light bulb, the differences between common threads and plugs, and what to watch out for when replacing halogen bulbs with LEDs.
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