The hinges are used to connect the moveable control surfaces of the aircraft to the fixed surfaces and allow smooth easy movement They may take several forms including hinge points, pinned hinges, "living" hinges, etc
The value of the lower hinge is approximately the first quartile, or 25th percentile The value of the upper hinge is approximately the third quartile, or 75th percentile The values of the hinges and quartiles may differ slightly because of differing computational conventions