The filament needs two connections for electricity to come in and go out. These connections are made through the eyelet and the shell whcih make up the screw base shown on the right.

Screw bases are common but some non-screw bases like end-prong and bayonet bases are available on certain lamps.

The standard household bulb has a medium base. The mogul base is larger in size and current carrying capacity and is used in higher wattage lamps.