Light fixtures and light bulb sizes

The Edison screw (E..) is a standard socket for light bulbs developed by Thomas Edison. There are a lot of exotic light bulbs however in practice only a few common types are used.

Light bulbs with an Edison screw

On the packaging of bulbs for household lighting you will normally find one of the following "size" designations.

On the package of European light bulbs you will normally not find any numbering describing the shape or diameter of the glass. The light bulb is specified only by its base and there will be photo showing the shape.

Four of the most common light bulbs: US A19 with E26, European E27, US B10 with E12, European E14

The E27 light bulb has slightly deeper grooves in the thread than the E26 bulb base but the US light fixtures are less precise and have more slag than the European ones. This results in the odd situation that most E27 light bulbs (the supposedly bigger ones) will still fit well into a US E26 base but not the other way round. You can however force it, it may feel tight and difficult to turn but it will work.

Light bulbs and light sockets were in the past produced locally. With the decline of incandescent light bulbs and the raise of more expensive LED bulbs the production of those bulbs is moving to China. Those China factories are producing for the world market and the result is a base that has the outer diameter of a E26 base and groove depth of a E27 bulb. The result is that LED bulbs fit now easily into either a European fixture or a US standard fixture. The box will still say E26 or E27 depended on the market but the light bulb will fit in either fixture and you just have to make sure that the light bulb can take the voltage.

British light bulbs with a bayonet cap

"push and twist" light bulbs with a base known as Bayonet Cap (BC) are fairly common in Great Britain but used nowhere else in the world for domestic lighting.
Since this British bayonet cap is used exclusively in Britain it becomes a problem when you are moving to another country. You can find now adapters that allow you to use standard E14 or E27 light bulbs in B22d fixtures.

Adapter to use light bulbs with an E14 base in British Bayonet Cap B22d light fixtures.

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