Abstract: A suitcase, lid or door has a first polygonal part and a second polygonal part. The first and second parts are engaged against each other in a closed position for the assembly. The first and second parts each having at least two non-parallel sides connected to each other at a corner and extending at an angle to each other. Each side has a rabbet edge and the rabbet edges of the first part are shaped to closely engage the rabbet edges of the second part in the closed position. The edges of each side have a gently undulating profile with at least one arc on each side, one of the parts having convex arcs which correspond to, and closely engage concave arcs of the other part. The corners are rounded and one of the concave arcs extends around each corner.