Many people today are looking for improved lighting output from their car's stock halogen projector headlights. However, if you're not an experienced "retro-fitter " it becomes difficult to navigate at times. Take an H7 bulb for example.. You've determined that the bulb needed for your car is an H7, but then you realize that said bulb comes in both H7A and H7B. HELP!!!
So What's the Difference?
The socket of the headlight housing will have three reliefs or cutouts for these tabs to be inserted. One tab is wider than the other two. It is the orientation of this widest slot on the headlight housing that will determine which bulb you will need. See the picture below for reference. Looking at it from the front, the filament should be on the bottom of an H7A bulb and located on the top of an H7B bulb.
Why Should I Care?
If you use the wrong bulb, the return wire on the HID bulb will be at the top once installed in the headlight. The return wire will cast a shadow down the middle of the car's light output, causing the reduced performance of the light. It's not the end of the world, but you won't be getting the best light output possible.
How Do I Determine What I Need?
The answer is simple. You need to look at the back of your headlight (though this is easier said than done sometimes). Either light will do, both will be the same. Note the orientation of the widest slot, then place your order accordingly. That's it!