The cap/base of a bulb is the part that connect it to the socket in the fixture. E27 (Edison screw) and E14 (small Edison screw) are among the most popular, although this depends on your need. GU10 is generally the most common cap/base for spotlights. Check which cap/base you need carefully as many other types are also available.

Here you can read more about the most common sockets.

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