Limited Degrees of Freedom
"Limited Degrees of Freedom" is a concept in design where positioning is determined by the minimum number of contact points. Only as many contact points are used as is needed to insure the accurate positioning of a part.
For a simple illustration of this concept, let us consider the design of a bar stool. If a bar stool has four legs, as many do to support the additional weight of some beer drinkers :-) , it would rock back and forth on an uneven floor. If the bar stool had three legs, it would stand in a stable fashion even if the floor was uneven. Therefore, under the principles of Limited Degrees of Freedom, to make a stable bar stool for any floor surface, it should be designed with three legs.