Wedge base

The bulb is inserted and removed with straight in or out force, without turning as with a bayonet mount or Edison screw, on certain bi-pin light sockets.

For true wedges, compression is the force that holds the bulb in, while others use simple friction, or snap into a socket with spring-loaded electrical contacts that "grab" the corners or other protrusions on the base to prevent loosening from vibration.

Along with putting contacts on the opposite (alternating) sides, this gives the base rotational symmetry so that the user does not have to determine the exact position of the bulb before inserting a new one.

Modern sets typically have a long and thin wedge that extends all the way down to the back end of the socket, separating the two electrical conductors and contacts and preventing water between them when used outdoors.

This is problematic for light sets with covers like icicles, which will not fit over this type of base or socket unless they have a special notch, which can in turn allow rainwater or snowmelt into the decoration.

A small resistor may also be contained in the base of older two-volt LED colors (red, orange, yellow) when in a mixed-color set, so that they match the three volts needed by the newer colors (blue, deep green, purple, white), however other manufacturers only change the value of the resistor that is a part of the cord set itself.