B22 Bayonet Cap With its familiar “push and twist” action, “bayonet cap” (also known as BC or B22d) is used on most regular light bulbs. It is 22mm diameter and with two locating lugs. The “small b22 bayonet cap” (SBC or B15d) is very similar but only 15mm across.

The traditional B22D bulb is used at home in traditional light fixtures. A small-cap version, or SBC, can be found in smaller applications. Double-contact versions are for home lighting while single-contact models are for automotive or machine use. Small caps, or MBC, are usually used for industrial applications.