The changing plans for St. Peter's. The architectural terms are explained in the article.
When Pope Julius decided to build the "grandest church in Christendom" the design by Donato Bramante was chosen, and Pope Julius laid the foundation stone in 1506. Bramante's plan was in the shape of an enormous Greek Cross, which means that it had four arms all of equal length, and a large dome at the middle. For the next hundred years, the "groundplan" got changed backwards and forwards between a "Greek Cross" like Bramante's plan and a "Latin Cross" like the old basilica, but one thing never changed, and that was the idea of having an enormous dome at the place where the two arms crossed.
At that time, there were only three very large domes in the whole world. One was far away in Constantinople on the church of Hagia Sophia and not many people in Italy had seen it. The other two domes were both very well known. One was the dome on the temple to the Ancient Roman gods, called the Pantheon. The other dome was built in the early 15th century (1400s) on Florence Cathedral by Filippo Brunelleschi. The dome of the Pantheon is 43.3 metres (142.06 ft) across and the dome of Florence Cathedral is about 42.1 metres (138 ft), but is much taller. Bramante's plan for the dome of St. Peter's was for it to be about as wide as the dome of Florence, and even taller.
No architect with any sense would try to design a dome without first checking out how these other two domes were made. Bramante checked them out. He discovered that the dome of the Pantheon, which had been standing for nearly 1500 years, was made of concrete. So that the concrete wouldn't be too heavy, it was mixed with pumice stone which comes out of a volcano and is full of gas holes so it is very light weight. Bramante learned how to make concrete like the Ancient Romans.
Bramante's dome was to be like the one on the Pantheon. But there was one very big difference between the Pantheon dome and Bramante's design. The Pantheon's dome stands on a round wall like a drum, with only one doorway in it, but Bramante's dome was designed to stand on a drum, which was standing high up on four wide arches. The aches rested on four enormous "piers" (pillars of stone). He had got this idea from Florence Cathedral which had an enormous dome resting on eight big piers. Another idea that Bramante got from Florence Cathedral was the design for the little stone tower which sits on top of the dome and is called the "lantern".
When Pope Julius died in 1513, the next pope, Leo X, called in three architects, Giuliano da Sangallo, Fra Giocondo and Raphael. Sangallo and Fr Giocondo both died in 1515. Raphael made a big change to the plan. Instead of having a Greek Cross, he decided to change the plan to a Latin Cross, which had a long "nave" and "aisles" like the old basilica.
Raphael also died, in his mid-30s, in 1520, before any important changes could be made to the building. The next architect was Peruzzi who like some of the ideas that Raphael had, but did not like the Latin Cross plan. Peruzzi went back to Bramante's Greek Cross plan. But there were so many arguments in the church that the building stopped completely. Then in 1527 Rome was invaded by Emperor Charles V. Peruzzi died in 1536 without his plan being built. The only main parts of the building which had been constructed were Bramante's four big "piers" to hold the dome.
Antonio da Sangallo (known as "Sangallo the Younger") looked at all the different plans by Peruzzi, Raphael and Bramante. He put some of their ideas together in a design that had a very short nave, (not a long one like Raphael's design) and had a big porch at the front. He changed Bramante's dome to be much stronger and also much more decorated. The main new idea that he added were 16 stone ribs to strengthen the dome. This idea came from Florence Cathedral which had eight stone ribs. But Sangallo's plan never got built, either. The main job that he did was to strengthen Bramante's piers which had begun to crack.
On January 1st, 1547 in the reign of Pope Paul III, Michelangelo, who was already over 70, became the architect of St. Peter's. He is the main designer of the building as it stands today. Michelangelo died before the job was finished, but by that time, he had got the construction up to a point where other people could get it finished. Michelangelo had already done a lot of work for the popes, carving figures for the tomb of Pope Julius II, painting the Sistine Chapel ceiling, which took five years, and the enormous fresco the "Last Judgement" on the wall of the Sistine Chapel. Michelangelo found the popes and the cardinals very difficult to work with. When Pope Paul asked him to be the new architect for St. Peter's, Michelangelo did not want the job. In fact, Pope Paul did not really want Michelangelo. But his first choice, Giulio Romano, died suddenly. Michelangelo told the pope that he would only do the job, if he could do it in whatever way he thought was best.
Michelangelo wrote:
When Michelangelo took over a building site in 1547, the nave of the old basilica was still standing and in use. There were four of the most enormous piers in the world standing where the western part of the old basilica had been. The building work had stopped for so long that weeds and bushes were growing out between the stones of the unfinished building as if it was a cliff. Michelangelo looked at all the plans that had been drawn by some of the greatest architects and engineers of the 16th century. He knew he could do whatever he liked but he had respect for the other designers, especially Bramante. He knew that he was expected to make a design that would be the symbol of the city of Rome, in the same way as Brunelleschi's dome was the symbol of Florence where Michelangelo had lived as a young man. He went back to the Greek Cross idea and re-drew Bramante's plan, making every part of it much stronger and simpler. It had to be strong enough to support the tallest dome in the world.
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.
As it stands today, the "Greek Cross" part of the basilica is Michelangelo's design and the "nave", which was added later, is by Carlo Maderna. Comparing Michelangelo's plan with Raphael's plan shows that while the outside-line of Raphael's plan has clear square and round shapes, the outside-line in Michelangelo's plan has lots of changes of direction. That is the way it was built. All around the outside of the building are enormous "pilasters" (which are like giant columns stuck on the building). Almost every pilaster is set at a different angle to the next one as if the flat walls had been folded up. Right around the top of the building is a band called the "cornice". A "cornice" is usually quite flat, but because of all the changes of direction, this cornice ripples like a giant piece of ribbon, tied around the outside of the building. The art historian Helen Gardner wrote that it looked as of the whole building was being held together from top to bottom.
Michelangelo designed the dome again, using ideas from Bramante and Sangallo the Younger. Three important ideas came from the dome that Brunelleschi had built in Florence more than 100 years earlier.