Doors let people into spaces. That's what people are looking for when they come up to a door, whether it's made of wood, metal, glass, or some unknown material: Where is the handle, and how do I use it to unlatch and push open the door? As Don Norman has pointed out in his book The Design of Everyday Things (1988), even such an obvious and simple user need is easily defeated by poor design.

Norman (1988, 3 4) illustrates his point with an amusing anecdote of a friend "trapped" in the doorway of a European post office. The door in question was part of an outside entryway (a row of six glass doors), with an identical internal entrance beyond. As the man entered through the leftmost pair of doors, he was briefly distracted and turned around. The rotation caused him to slightly shift his position to the right. When he moved forward and pushed a door in the next row, it didn't move. He assumed it must be locked, so he moved to the next pair of doors. He pushed another door; it also refused to move. Beginning to feel confused, he decided to go outside and try again. But now when he pushed the door leading back outside, it also didn't move. His confusion turned to mild panic . . . he was trapped! Just then a group of people entered at the other end of the doorways. Norman's friend hurried over and followed them, and was successful...