Style A base 3.5v .425 watt mini light bulb flashers to make your lights flash. You must match the flashers to your replacement bulb voltage and wattage or they will not work. These flashers work in light strings that use a 2.5 volt 0.425 Watt Bulbs. Using the incorrect flasher bulb will cause it to burn out quickly or not work. You can remove these from the base to use on any string with the correct voltage and wattage. Please check pictures for a measurement diagram for the style A base. Incandescent indoor/outdoor bulbs. These are commonly used on 100 and 50 light strings. Will not work for LED bulbs, these are incandescent mini lights.