Typically skip bins have a distinctive shape: the longitudinal cross-section of the skip bin is either a trapezoid or two stacked trapezoids. The lower trapezoid has the smaller edge at the bottom of the skip bin, and a longer edge at the top. The smaller edge on either end is lower which makes it easier to load. Where there is an upper trapezoid, it has the smaller edge at the top. At either end of the skip bin there is a sloping floor or wall. There are usually two lugs on each side of the bin onto which chains can be attached, permitting the heavy skip bin to be lifted onto and off a skip lorry or skip truck. A special skip-carrying lorry or crane is used.