This light set has been carefully manufactured and pre-tested to give you complete satisfaction , It is a replaceable push-in series set and will not light if even one bulb is removed from of loose in its socket. However, should one bulb out(but remain in its socket)the other bulbs will continue to light. Nevertheless, to prolong the life of your set, replace the burned out bulb as soon as possible.Use spare bulb provided with the package.Do not twist or tum the bulbs when removing or replacing. They are of delicate construction and may be damaged by rough bending.