As written elsewhere, these caps fit over standard white H4 bulbs, and are better than the yellow bulbs because not only do they look more yellow, especially when switched off, but because once you've got a pair, you can simply take the cap off a blown bulb and put it on the new (white) bulb. Much cheaper, and means you won't have to drive round with one yellow and one white bulb until you order more yellows.
Usually these caps are around £16 for 2 with postage, but I just bought EIGHT for €24 with postage - they're €2.50 each, which can't be bad!
I have a set of yellow bulbs in my BX, but these caps look better - hence my order when I found them cheap.
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