A trullo is a house built with a conical roof that will collapse completely with the removal of a keystone. In the Middle Ages, the local Prince was taxed annually on the number of standing homes in his principality. To avoid paying the tax, he demanded that homeowners tear down their domiciles every year, hence the trulli roofs. It was only in the late 1700's that a nobleman found out about this tax evasion, and ruled that the Prince allow his subjects to build permanent residences. The first such edifice is evident in the picture that follows.