Does a featherbed go on top of the bottom sheet or under it?

A featherbed goes under the bottom (fitted) sheet. Even better, to protect it, put the featherbed directly on top of the mattress, then put a mattress protector/pad on top of the featherbed, then the fitted sheet. The mattress pad can be regulary washed while many feather beds are not washable, even if made from synthetic materials.