E39 is known as the large base or mogul base. Its typically what is used on older metal halides and high pressure sodium from 100 watts and up. The E is for Edison screw base and the 39 is 39 mm across. Larger than the standard E26 or 26 mm across.

Bulbs with E12 bases are commonly used for decorative applications, such as chandeliers, wall sconces, Christmas lights, restaurant pendant lights, ornamental lights, residential or commercial string lights, and night lights. Many bulbs with these bases are designed to mimic the look of a candle flame. Wedge bases are designed to be easy to use and provide a secure connection. They can be found in a variety of sizes and configurations, depending on the specific application. The screw-in bases, Edison E26 and the larger-sized E27 are usually interchangeable. The medium-sized B22d is also often used with a bayonet fitting. There are dozens of different light bulb bases. The most common is the Edison screw base, which may be found on most incandescent bulbs and many halogens, compact fluorescents, HIDs, and now LED lights. Edison Screw BasesThe letter “E” in E26 refers to Edison Screw, the most standard of lamp bases, while the numerical “26” refers to its diameter in millimeters (approximately 1 inch). As the name suggests, the fitting is 12 millimeters in diameter size and is often deemed the Candelabra Edison Screw (CES). It is the home of C7-sized lamps, so if you bought a C7 lamp before, it utilized the E12 fitting. The biggest light bulb base is one with a diameter between 40mm and 50 mm (E39 or E40). Which is the smallest light bulb base? Halogen various wattage (e.g. 50 W/100 W), various voltage (e.g. 12/24 V). Common for task lighting, landscape lighting. In North America and continental Europe, Edison screws took the place of other types of bases for general lighting.

E11 and E12 are not interchangeable because of their different size. Does the Bulbs Base affect voltage and performance?

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Intermediate Screw Base (E17) Bulbs are sometimes called Intermediate Edison Screw (IES), “E” stands for “Edison” and “17” indicates the diameter in millimeters as measured across the peaks of the thread on the base When you buy light bulbs for a home with C-type outlets, make sure they can handle the higher operating voltages – otherwise, you risk having them blow out or catch fire if used in such fixtures. In addition to automotive lighting, wedge bases are also used in other applications such as outdoor lighting, commercial lighting, and theatrical lighting. They are a popular choice because of their simplicity, reliability, and versatility. COMPACT FLUORESCENT PLUG IN BASES The three most often used bases in the United States of America for traditional lighting are the E12 (candelabra), the E26 (medium), and the E39 (mogul). Light Bulb Base Sizes Chart