Beekeepers know that bees provide a space of around 6mm-9mm between their combs, and between the walls, floor, and ceiling of the cavity they occupy, to allow them to move around. Spaces and gaps that are too small are filled with propolis, those that are too large are filled with wax comb. When a box to contain the frames of comb is designed the maker must adopt the same rule and ensure this spacing is maintained. What a designer does is to place all the 'fault lines' or break points that allow boxes to be separated and frames to be removed within these natural 'corridors'. As hive boxes are designed to be stacked one on top of the other clearly one of the natural separation points must fall between the boxes, and so a bee space must be placed at the top or bottom of each box. The box is therefore said to be 'top bee space' or 'bottom bee space'. Beekeepers will claim each design has its merits, but the important thing is not to mix the two designs. If one box has the bee space at the top and a second box is added with its bee space on the bottom (or vice versa) the interstitial space will be twice the bee space, or there will be no space at all! In either case the gap will be filled and prevent the boxes from being separated. You can tell which spacing has been used by running a straight-edge across the top of the box containing its frames. If the tops of the frames meet the straight-edge and are level with the sides there is no space, it must therefore be at the bottom. If the frames are lower than the sides and do not meet the straight-edge your space is at the top. Be particularly careful when buying lots of second-hand equipment that you do not end up with incompatible designs.