The Noctua NH-L9 series comes in two variants, the i and the a. Naturally the i is for Intel and the a is for AMD. The reason two separate models are required is because these CPU coolers are so unbelievably small, just 37mm high, that it is not possible to develop a universal mounting system. The shape of the i and a variants is determined by the layout of the Intel LGA 115X and AMD (AM2(+)/AM3(+)/FM1/FM2) sockets.



Link: eTeknix