I wrote this as a report for school in the spring of 2001. Accompanying it was a series of small models of various arches from different cultures and time periods.

The arch is an incredible architectural discovery, dating back to ancient times but still in wide use today, as, up until the 19th century, it was the only known method for roofing a building without the use of beams. It comes in many shapes-semicircular (Roman), segmental (less than half a circle), or pointed (Gothic). The arch developed from the post and lintel or possibly the corbel, which is similar in shape and principle to the arch. Efforts to build corbeled roofs with smaller units and less weight could have eventually led to the discovery of the arch.
Arches are made of wedge-shaped blocks, called voussoirs, set with their narrow side toward the opening so that they lock together. The topmost voussoir is called the keystone. Once locked into place, the arch cannot collapse under any amount of weight and the only danger is of the voussiors crumbling under the pressure. To keep this from happening, most arches require support from other arches, walls, or buttresses.
The arch has been found in many different cultures, as early as Mesopotamia. The Egyptians used it in tombs and vaults but never for monumental architecture, such as temples. They apparently thought it unsuited to this purpose. The Greeks also used the arch solely for practical constructions, but many of the principles they developed were later exploited by the Romans.
Overall, it was not until the time of the Etruscans that the arch was used in any kind of monument. The best example of this is the Porta Augusta, where the arch is combined with Greek architectural ideas. The Romans borrowed this combination and used it over and over again, but its invention belongs solely to the Etruscans.
The Romans took many great strides in the development of the arch. While they borrowed many techniques from earlier races, the Romans invented the idea of setting an arch on top of two tall pedestals to span a walkway such as a public highway. The outer wall of the Colosseum appears composed almost completely of arches. Here we see examples of the barrel vault and the more complicated groined vault, both developed by the Romans from the basic arch. The Romans also used arches for common purposes, such as in the building of bridges and aqueducts.
Arches continued to be used in Medeival times, especially in cathedrals, (above, second from right), where they helped support the great weight of the stone ceilings, especially when walls were weakened by the presence of many windows. It is here that buttresses were often used to support the arches. Sometimes called "flying buttresses" because of their height, buttresses are a simple construction of a stone pillar with a "bridge" at the top that joins onto the arch or walls of the building, giving extra support to the construction. Arches were also often found in long rows in cathedrals to help support each other. It is about this time that the pointed arch began to be developed, as an alternative to the traditional rounded arch. This pointed or Gothic arch became very prevalent in the architecture of the time.
Unique to architecture was the Islamic arch, found about the same time in the Middle East. Many advances were made in the arch by this culture as well. While the pointed arch was used here, the Muslims also developed a horseshoe-shaped arch and "stacked arches," an arch built above an arch. It is believed that the "stacked arch" idea developed by accident, when a builder was forced to use columns too short for his purpose and so stacked them on top of each other, with arches holding the stacked columns together. Islamic arches can be found in mosques throughout the Middle East.