If solving problems having a decomposition width bounded by a constant is in P, the decomposition leads to a tractable structural restriction. As explained above, tractability requires that two conditions are met. First, if the problem has width bounded by a constant then a decomposition of bounded width can be found in polynomial time. Second, the problem obtained by converting the original problem according to the decomposition is not superpolynomially larger than the original problem, if the decomposition has fixed width.