Minecraft wiki entry for Curved_roofs, associated text: Domes do not necessarily have to be circular domes. Other possibilities include ellipsoidal domes, bell domes, onion domes, surmounted domes, segmented domes, compound domes and more. With the exceptions of the surmounted and segmented domes, these other shapes tend to be harder to model well than a simple semi-circular dome, so unless you are working on a very large scale project and have computer assistance it's probably best to avoid them.