Constriant propogation is one way to partially solve constraint saisfaction problems. As soon as information is known about a variable, it is fed through into any constraints that refer to it. For example, if the set of constraints includes , A=B+C, then if we know B is in the range [1,3] and C is in the range [2,5], then we can propogae this into the constraint so that we know that A is in the range [3,8]. Sometimes this yields a solution on its own, but more often needs to be combined with other forms of search.
Used on Chap. 4: page 79