Efficiency has always been key in the design of container ships.[32] While containers may be carried on conventional break-bulk ships, cargo holds for dedicated container ships are specially constructed to speed loading and unloading, and to efficiently keep containers secure while at sea.[32] A key aspect of container ship specialization is the design of the hatches, the openings from the main deck to the cargo holds.[33] The hatch openings stretch the entire breadth of the cargo holds, and are surrounded by a raised steel structure known as the hatch coaming.[32][33] On top of the hatch coamings are the hatch covers.