Socket adapters are devices that screw into lamp sockets to allow light bulbs of different sizes to fit the lamp. Adapters may be used to make the lamp socket larger or smaller; these are known respectively as socket extenders and socket reducers. The adapters are available in different attachment designs to work with all types of lights, including Edison screw base, bayonet base and pin base attachments.
Edison screw attachments, which were invented by Thomas Edison, are the most common light bulb base attachment style in America. A screw bulb attachment twists into the lamp base; turning it to the right tightens the bulb, and twisting it to the left loosens it. The contact point of the bulb is located at the tip. Although the Edison screw design is the standard attachment type, light bulbs may come in different sizes. A socket adapter can be used to fit a different-sized bulb in a light fixture or to replace an incandescent bulb with a fluorescent model that would not otherwise fit.
Light bulbs with bayonet bases attach to light fixtures using two interlocking cylindrical connectors. One connector has radial pins that fit into slots on the second connector; the unit can then be twisted into locking position. Bayonet bulbs are often used in Europe and the United Kingdom; they are also commonly found in flashlights and automotive lighting, both in the United States and abroad. Socket adapters with bayonet-style tops and screw-type bottoms may be used to affix a bayonet bulb to a screw base. Manufacturers such as Satco, Shore Holders, Servalite and PLT design adaptors for this purpose.