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If we asked you to picture a lightbulb, the image that would likely be the first to jump to mind is that of an A19 incandescent bulb. The A19 has been largely unchanged since Thomas Edison cornered the market on DC power supplies and indoor illumination in the 1800s. The first incandescent bulbs used a filament strung between two vertical supply wires. As electric current ran through the filament, it would glow, giving off light.

The first light bulbs were a huge success. Initially prohibitively expensive, it didn’t take long for them to become a household staple. Unfortunately, even with modern improvements, incandescent bulbs are woefully inefficient. Only 5% of the electricity consumed is turned into visible light. The rest is given off as heat. As a result, traditional incandescent bulbs can get very hot to the touch and, if not used properly, can pose a fire risk.

Edison was so influential when it came to the commercialization of the light bulb that we still reference his name in the industry to this day. The utility provider ComEd is an abbreviation for Commonwealth Edison and the base for many light blubs today still bear his name in their designation. The “E” in the base classification for many standard bulbs stands for Edison; a reference to the base design he created.

There have been many advances in the manufacture of incandescent bulbs since Edison’s day. Even still, they are among the least efficient bulbs on the market. Compact Fluorescent Lights (CFL) and Light Emitting Diodes (LED) are many times more efficient and often don’t cost much more up front. They have the added benefit of being less expensive to run, leading to even more savings over the life of the product. While the technology has changed, the A designation is still used by these newer bulbs as they mimic the shape and size of their predecessor.

The most common size for the most common bulb is the A19. The “A” refers to the shape, whereas the “19” refers to the diameter at its widest point, and is measured in eighths of an inch. An A19 bulb (also designated A60 in countries where the metric system is the standard) has a diameter of approximately 2.4 inches. Because A19 bulbs typically utilize the extremely common E26 base, and are often the default bulb, most household fixtures not only accommodate them, but were designed around them. The A19 is almost certainly the most common bulb in your home. Replacing your existing incandescents with LEDs not only ensures each bulb lasts longer, they will use less energy, potentially saving hundreds of dollars on your utility bill over the course of the life of the product.