The three most commonly used light bulb bases in holiday lighting are the following: candelabra base (E-12), intermediate base (E-17), and medium base (E-26 or E-27). Candelabra bases are the smallest and medium bases are the largest. For each type of base, there is a corresponding socket size. For example, a light bulb with a candelabra base will fit only a candelabra socket. Therefore, when purchasing light strings, be sure to purchase bulbs with compatible bases. If you purchase light strings with Medium sockets, be sure that the light bulbs you purchase also have a Medium base.