Standard Bulbs - General Information
Older style bulbs have a metal cap and are available with cap diameters of 7, 9, 15, 20, 21 and 22mm. 7 and 9mm bulbs are generally used in warning lamps, 15mm in side, indicator and brake lights and 15,20,21 and 22mm bulbs in headlamps.
Bulb names show what type of bulb they will be:
For example: a BA15s bulb will have a bayonet fitting with parallel pins, a cap of 15mm diameter and is single contact.