Flex top King Sheets Also Known as Head Split or Top Split King Sheets are made for special king size mattress where the top half of the mattress is split and the bottom have is not. This special mattress is sold by different bed manufacturers including Sleep Number Bed, Reverie and many others. The Head Split into the middle of the upper portion of the mattress is about 32" and fitted sheets are made with 6 pockets to properly fit this mattress.