Put a decoration in the middle of the arch to increase artistic beauty and visual appeal. This ornament is called "keystone" or "keystone" and is located at the top and center of the arch. The keystone plays an important role in the structure. By adding extra support at the highest point of the arch, it helps to disperse and bear the weight above, thus increasing the stability and durability of the arch. In addition to structural functions, keystones are often used as decorative elements to improve the aesthetic value of buildings. It can be beautifully carved stones, patterns, reliefs, etc. It shows the design talents of architects and artists and adds unique style and charm to the whole building. In a word, placing decorations in the middle of the arch, such as keystone, not only provides the necessary structural support for the building, but also increases the aesthetic feeling and artistic value of the building. This design makes the arch a practical and artistic architectural element.