For many people today, one of the things they want from their car's stock halogen projector headlights is improved lighting output. The original bulb used is an H11 style bulb. However, there are actually two different mounting styles for these H11 bulbs; H11A and H11B. The difference is in the orientation of the three little tabs around the circumference of the bulb's base.
What's the difference?
The socket of the headlight housing has three tabs for inserting a bulb. One tab is wider than the other two. It is this orientation of the widest tab that will determine which bulb you need when changing a burned-out headlight.
What if I buy the wrong one?
Using the wrong bulb will leave the return wire on top of the bulb, casting a shadow down the middle of your light output. You won't be getting the best light output possible.
How do I know if I have H11A or H11B?
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!
Pro Tip
Its important to note this only applies when shopping for HID bulbs. If you are buying LED bulbs there is no difference between H11A or H11B because may led bulbs allow adjustable orientation and also do not have a return wire that runs the length of the bulb like the HID bulbs do.