Identify the Base of Your Bulb
Now that you've determined the size and shape of the bulb you're looking for, you'll need to find the right size base to fit the socket in your fixture. Bulb bases are measured in millimeters (as opposed to the bulb size, which is measured in eighths of an inch).
Screw Bases are noted by the letter E, which stands for Edison, followed by the size of the base in millimeters. A standard A-19 bulb has a medium E26 base, which is 26 millimeters in diameter.
Bi-Pin Bases have two contact points which protrude from the base. The diameter of a Bi-Pin base is measured as the distance in millimeters between the two pins. For instance, a GY6.25 base has a distance of 6.25mm between the two pins.
Bayonet (or push-twist)
Bayonet (or push-twist) bases are used with sockets with spring loaded base plates. The wiring wraps around the outer side of the base, and makes contact with the conductor plates within the inner sides of the socket. A prime example of a small bayonet base is a miniature holiday light (see slide #1 and slide #2), which slides into the socket, transferring electricity through the wiring that runs along the outer sides of the plastic base. Bayonet bases are commonly used in European fixtures.
Wedge Bases operate in a similar manner to Bayonet bases, but are constructed of glass or plastic.