Bingchen (丙辰) and Dingsi (丁巳) belong to the "Sand in the Soil" category. The Earthly Branch Chen (辰) is the storage of earth, and Si (巳) is its extremity. The Heavenly Stems Bing (丙) and Ding (丁) represent fire, which reaches its peak at Chen (辰) and is at its official position at Si (巳). Since the earth is stored, exhausted, and prosperous, and the fire further nourishes it, it is called "Sand in the Soil."