You can't place a round dome upon a cube just by this. A cantilever construction is a better alternative. The angles of the cube are truncated to create an octagon base that's more adequate to sustain the round dome.

The change of the shape from square to octagon can be realised layer upon layer using a cantilever construction: layer upon layer is protruding.[br]In the applet below you can see how this is realised in three layers. You can follow this construction using the buttons of the navigation bar.

In Islamic architecture as well in romanesque architecture you can see this stepwise transition from square to octagon in many variations. The picture below showq an 8th century basilica in Odzun (Armenië).[br]In Islamic architecture this cantilever constructuion gradually develloped to muqarnas. DIfferent regions claim the origin of muqarnas, wich is mostly situated in the 10th century.[br]Muqarnas rapidly became a typical aspect of Islamic architecture. Typical are the semi-domes in iwans (large entrance gates) and mihrabs (prayer niches) in mosques.