The difficulty of dealing with a novel situation is directly related to the number of possibilities.
Constraints are physical, semantic, cultural, and logical limits on the number of possibilities.
Physical constraints such as pegs and holes limit possible operations.
Semantic constraints rely upon our knowledge of the situation and of the world.
Cultural constraints rely upon accepted cultural conventions.
Logical constraints exploit logical relationships. For example a natural mapping between the spatial layout of components and their controls.
Where affordness suggest the range of possibilities, constraints limit the number of alternatives.