Collection: BA15D Bulbs

BA15D: The symmetrical double-contact not to be confused

The BA15D base (bayonet 15 mm, 2 symmetrical centered contacts) is often confused with the BAY15D (P21/5W) which has its 2 pins at different heights.

Crucial difference:

• BA15D — 2 pins at the same height (symmetrical). Can be inserted in any direction. Used in some dual-intensity brake lights on old vehicles and towing applications.
• BAY15D — 2 pins at different heights (asymmetrical, the "Y"). The offset prevents reverse mounting — important for the brake+position function.

BA15D vs BAY15D: Impact on use

On a light where the tail light and brake light are distinct but use the same double bayonet base, the BA15D may be suitable. For modern lights combining both functions, the BAY15D is essential — its anti-reverse feature ensures that the 21W filament is dedicated to the brake light.

Identify your exact base

To distinguish BA15D and BAY15D: count the pins and check their height. Place the bulb on a flat surface:

• 2 pins aligned at the same distance from the glass = BA15D
• 2 pins at different distances (one closer to the glass than the other) = BAY15D

If in doubt, compare it to the original bulb or consult the vehicle manual.