 Each lever is engaged with the interlocking logic such that movement of the lever is only possible when all necessary conditions are met.The interlocking may be mechanical, electric (via solenoids) or both with the apparatus being mounted horizontally behind the lever frame or vertically below it.To assist the operator in determining their functions, each lever in a frame will generally be uniquely labelled, one common method being to number the levers in order from left to right.