When you want to reach from one side of a structure to another you can use a straight line or an angle or a curve. A curved element that spans across a structure is an ARCH. This element also must support weight. Sometimes an arch makes the top of a doorway or the top of a window and often an arch is the shape of a roof.
Arches have been around for an incredibly long time. Mesopotamians used them in their building and then the Romans used them all the time. And they are still used today.
Let’s consider arched roofs for a minute. Before the arch, a roof could only be as wide as a straight beam that would attach the two opposite walls. If you had a piece of wood that was 10 feet long, then the largest room you could build would be 10 feet wide. That was not convenient for someone who wanted to build a large space, like a church, or the Colosseum in Rome.
Also, an arch carries weight from above and shares it efficiently to the walls below. (It is called “weight distribution.”) So think about this. If you have a flat roof, built with beams and you put an elephant on top, the roof might break and the elephant would fall.
If you have an arched roof and put an elephant on top, the arch “distributes” the weight down to the walls and then the elephant stays outside, without squishing your couch inside. Arches are very effective.
Try building a model of the Colosseum yourself.