An A-series light bulb has a tubular base with a round top, according to eBay. This standard-shaped light bulb, provides incandescent light and is used largely in household ceiling lights and lamps.
The pear-shaped A-series light bulbs range from A-15 and A-19 to A-21 and A-23 models, says Bulbs.com. The number after the A is the diameter of the bulb in eighths of an inch, making A-23 the largest standard bulb size. The CP and PS light bulb series somewhat resemble A-series bulbs. The CP, or crystalline pear, series includes pear-shaped candle bulbs with crystalline surfaces that are used in decorative fixtures. The PS, or pear straight, bulbs most closely resemble A-series bulbs with pear-shaped tops and tubular bases, states eBay. The PS series is also known as elongated standard incandescent bulbs and have longer ends, explains EagleLight LED University.
Some other common light bulbs are found in the B and C series, according to EagleLight LED University. Both types of bulbs have smaller tops and bases and bulging middles. Some of those tops are pointed or bent. These bulbs are used in low-wattage fixtures, such as holiday light strings and nightlights. They are also used in chandeliers with multiple lights.