 Later sets used nine bulbs on a string to increase the life of the bulbs by reducing the voltage each lamp received (120 volts divided by 9 lamps = 13 volts per bulb) but not significantly reducing the light output of the bulbs.Large C7½ and C9¼ bulbs typically come in sets of 25, though bubble lights come in sets of seven, and some non-holiday sets come in ten or twelve.Sockets are usually spaced about one foot (30 cm) apart, and are clamped to the wire with an integrated insulation-piercing connector.