a key blank , key and master keying system for a twisting tumbler sidebar lock of the type where the sidebar is blocked or unblocked by a selectively positioned longitudinally extending slider , has a key blank with a slider contacting surface on a rib on the blank below the bitting area at one of a plurality of selected predetermined locations . a key with skew cut bittings can be cut on the blank and the blank can be configured with the slider contacting surface at different predetermined locations . the slider contact area on the rib of the key blank can be divided into two horizontal portions at different predetermined locations and the slider itself can have contact surfaces also positioned at different predetermined locations so that a number of different master keying variations are possible .