Marked key / Marked key

Many people forget whether or not the door is locked after leaving home, and then go back home and check up. “Marked Key” could show whether or not the door is locked. The actions of locking or unlocking the door will put different marks on the key. So people could confirm whether or not the door is locked. “Marked Key” is especially helpful for forgetful senior citizens and people with obsessive-compulsive disorder.