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Many glass doors of most buildings are closed for energy saving and security reasons.
But when necessary, these doors are opened occasionally, and it is very annoying to change the mark plate showing the state of the door. (Because the state of door is obscure, people push or pull the closed doors.)
Mark and lock shows closed sign when you push the lever up, and shows open sign when push it down.
Also, because locking apparatus is located at the center of door, we can close and open door easily comparing doors prevailing nowadays.