If you look carefully at Figure 2112, you see that the cylinder has a top but the triangular prism and the box don’t. You get top and bottom surfaces on some objects with thickness, but not on others.
Če si pozorno ogledate sliko 2112, lahko opazite, da ima valj pokrov, trikotna prizma in kvader pa ne. Na nekatere predmete z višino sta postavljena pokrov in dno (zgornja in spodnja površina), na druge pa ne.