Medium level. Mid-levels were called Mid-levels in ancient times, which came from the Tang Dynasty. Liu Changqing's "Farewell to yang yuling to Bianzhou in Song Dynasty", "The rain in the mountain stream is oblique, and the guest clothes are scattered to the water. In ancient times, Chinese words meant the past, a long time ago, and historical stages, referring to the primitive commune system era and the slave era. In the historical stage of China, they generally refer to the era before the middle of19th century.