Binary Constraints:

Alternatively unary constraints specify that a particular variable can take certain values that basically restricts the domain for that variable thus should be taken care of where specifying the CSP.  Whether Binary constraints relate two variables, and to binary constraint problems  are individual CSPs that involve only binary constraints.

Whenever Binary CSPs have a special place in the theory than all CSPs can be written as binary CSPs as we don't go into the details of this here, and while it is possible in theory to do so there in practice the translation is hardly ever used. In fact there binary CSPs can be represented both graphically and utilizing matrices that can make them easier to understand. 

However Binary constraint graphs like the one below afford a nice presentation of constraint problems in which the nodes are the variables and the edges represent the constraints on the variables between the two variables joined through the edge as keep in mind that the constraints state that values can be taken at the same time. 

Binary constraints can also be represented as matrices, with a single matrix for each constraint. For example, in the above constraint graph, the constraint between variables x4 and x5  is {(1,3),(2,4),(7,6)}. This can be represented as the following matrix.