A system includes a first apparatus and a second apparatus. The first apparatus includes a first surface, a depression in the first surface, and a first connector disposed on the first surface. The depression has a bottom with a hole through which a screw for assembling the apparatus is passed and accommodates the head of the screw. The second apparatus includes a second surface facing the first surface when the first apparatus is detachably attached, a second connector disposed on the second surface and engaging with the first connector, and a protrusion fixed into the depression when the first apparatus is attached.