A door that won't stay put, when part way or all the way open can be annoying. Whether it swings closed or open, if it moves on its own, it is not properly adjusted. The problem can result from the door and it's hardware, or it can result from settling of the building.
If you hold a string with a weight tied to the end, the weight will swing and stop, pointing straight down. It stops at the lowest point of its swing arc. When a door moves, it is acting in a similar manner, it is trying to move to the lowest point in its arc. When a door is properly adjusted, all points of its swing arc are level and so there is no lowest point.
Assuming that a door is square and balanced, then it will have a low point in it's swing arc when one of it's hinges is not plumb with the other hinge. Using the the image below as an example, the top hinge is farther into the doorway than the bottom hinge. In this case the door will tend to swing closed. If the bottom hinge is farther into the doorway, the door will tend to swing open.
