 If a theory is complete, it is decidable (proof sketch: given a sentence \(A\), systematically generate the theorems of the theory; by completeness, eventually either \(A\) or \(\neg A\) will be produced in a finite time). The converse, though, does not always hold: there are incomplete theories which are decidable. Nevertheless, incompleteness at least opens the possibility of undecidability. Moreover, all theories which contain Robinson arithmetic Q (either directly, or Q can be interpreted in them) are both incomplete and undecidable. Thus, for a very wide class of theories, incompleteness and undecidability go hand in hand.