Light Bulb Shapes
Light "bulbs" come in a variety of shapes in addition to the classic light bulb shape. The classic light bulb with the spherical end (containing the filament) is has shape code "A" (for arbitrary) followed by the 8 x the major diameter. For the typical "A19" light bulb, this works out to 19/8 = 2-3/8" as the approximate major diameter. The diameter is important because lampshades often are designed to fit A19 bulbs and using a smaller size could affect how well the bulb stays in place or at all.
See Incandescent Lamps for more information.
Light Bulb Bases
Screw-in bulbs in North America typically come in 4 sizes. The "E" in the size code refers to the bulb's Edison thread (originally patented by Thomas Edison) followed by 2 numbers. Unlike bulb sizes, these number refers to bulb base sizes that are in mm.
| Name |
North American Size Code |
European Size Code |
Typical Applications |
| Candelabra | E12 / C7 | E11 | candelbras, decorative fixtures, night lights, Christmas lights |
| Intermediate | E17 / C9 | E14 (Small ES, SES) | appliance lights |
| Medium | E26 (MES) | E27 (ES) | common light fixtures, lamps |
| Mogul | E39 | E40 (Goliath ES) | industrial lights |
Compact Fluorescents
Stay away from buying CFLs, even if they're still cheaper than LEDs right now. From my personal experience, CFL bulbs do not last nearly as long as the incandescents they replace and burned-out florescent bulbs/tubes are hazardous waste (see EPA CFL Recycling and Disposal) because of the mercury they contain.
CFL can last longer than incandescents under the right conditions - that is, being left on for extended periods. CFLs burn out quickly if switched on for very short periods (like a 5 minute on/off cycle), which is how many people use their lights.