In recent years, much of the emphasis by the quay crane and spreader manufacturers has been on the development of multi-lift systems. Originally this was simply twin-lift, where two 20' containers are lifted at the same time as if they were a single 40' box. Now the equipment is available to do tandem-lift, two 40' containers side by side, and double twin-lift, four 20' containers arranged as though they were two 40s. There are also technologies available to handle several containers one above another but structural limitations in the container itself means that this can only be done with empty boxes.