Central plan: This architectural type was widely used during the early Christian period. The first central plan building appeared as a hall in Roman baths. Later, it was an antechamber in royal architecture. During the 2nd century A.D. it became very important due to the Roman Pantheon. Around the 3rd century it was used as a mausoleum because it symbolizes heaven. It is distinguished in circular, octagonal, hexagonal, three-conch and tetra-conch according to its form.