Roman architecture differed from Greek because of the discovery of concrete, and the development of arches and vaulting. A good example of this is the Pantheon which you can see to your left. 

Thanks to these innovations, from the first century C.E. Romans were able to create interior spaces that had previously been unheard of. Romans became increasingly concerned with shaping interior space rather than filling it with structural supports. As a result, the inside of Roman buildings were as impressive as their exteriors.

Roman concrete was developed early in the 2nd century BCE.  The use of mortar wasn’t new in the ancient world; mortar was a combination of sand, lime and water in proper proportions. The major contribution the Romans made to the mortar recipe was the introduction of volcanic Italian sand (also known as “pozzolana”).  The Roman builders who used pozzolana rather than ordinary sand noticed that their mortar was incredibly strong and durable. It also had the ability to set underwater. Brick and tile were commonly plastered over the concrete since it was not considered very pretty on its own, but concrete’s structural possibilities were far more important. Since concrete takes the shape of the mold or frame it is poured into, buildings began to take on ever more fluid and creative shapes.

The Romans also seized opportunities afforded to architects by inventing the true arch (as opposed to a corbeled arch where stones are laid so that they move slightly in toward the center as they move higher). A true arch is composed of wedge-shaped blocks, with a key stone in the centre holding them into place. In a true arch, weight is transferred from one stone down to the next, from the top of the arch to ground level, creating a sturdy building tool. True arches can span greater distances than a simple post-and-lintel. The use of concrete, combined with the employment of true arches allowed for vaults and domes to be built, creating expansive and breathtaking interior spaces.

To the right you can see a graphic representation of a barrel vault, which is essentially a long arch built as a tunnel. This graphic shows how the weight is distributed evenly across the vault. Graphic from Wikimedia CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1395490

Look at the graphic of the arches below. The true arch is to the left while the old corbeled arch is to the right. Look closely to see what makes the true arch hold weight more evenly. Can you locate the keystone?