An early structural restriction (that later evolved into that based on induced width) is based on the width of the primal graph of the problem. Given an ordering of the nodes of the graph, the width of a node is the number of nodes that join it and precede it in the order. However, restricting only the width does not lead to a tractable restriction: even restricting this width to 4, establishing satisfiability remains NP-complete. Tractability is obtained by restricting the relations; in particular, if a problem has width formula_29 and is strongly formula_29-consistent, it is efficiently solvable. This is a restriction that is neither structural nor relational, as it depends on both the scopes and the relations of the constraints.