Michelangelo was a sculptor. When he was going to carve something, he would start by making a clay model. Michelangelo could imagine the building like a lump of clay. What if the building could be pushed and pulled and squeezed? If you could squeeze the corners in, then other bits would bulge out. If you could put your hands around the whole building and squeeze it, then the dome would bulge upwards. The idea of imagining buildings as bendy and bulgy was a completely new one. But other artists like Gianlorenzo Bernini looked at what Michelangelo did at St. Peter's and used this clever new idea in their own work. This is called the Baroque style.