What does the middle door in the wooden door mean?

Actually, it's the middle door, that's all.

The door between the inner door and the outer door. "Li Zhoutian Yan Guanren": "The door is forbidden in the palace." Zheng Xuan's Note: "The central gate is the center from the outside, if the palace gate is closed. Thinking about the agricultural movement, Wang has five doors: the outer door is Gaomen, the second door is Luomen, the third door is Kumen, the fourth door is Yingmen, and the fifth door is Liumen ... Xuan is Luomen, and the third is also. " Sun Yirang's justice: "This door is not exclusive to pheasants. When the library, pheasant, should be three words. Cover five doors with Lumen as the inner door, Gaomen as the outer door, and the other three doors are between the inside and the outside, so it is called the middle door. " "Twenty-five Years of Zuo Zhuan Zhao Gong": "Outside her home." The history of Kun's correction: "Cui Jian's wife Zheng arrived and Wang sent her ... into the small pavilion outside Wang Zhongmen."