T25 refers for the base of the bulb. It comes in different types like 3156, 3157 and available in many colours. 3156 being a single beam type is usually used as a DRL (daytime running light), reverse light or marker light. 3157 is a dual beam bulb either used as a turn signal with the dual intensity as well as brake lights that are combined with turn signals. For those vehicles that uses the same bulb as the front DRL as the turn signals, we also have dual colour bulb that are default white on DRL mode and goes yellow when flashing to turn. It adds a nice unique touch for a complete white front end look while still being a 100% legal yellow turn signal.