The main concept of this is a stable, inverse relationship between inflation and unemployment. The short-run Phillips curve has a negative slope; the long-run Phillips curve is vertical. The Adaptive Expectations Theory predicts that there is a short-run tradeoff between inflation and unemployment but there is no long-run tradeoff. In the short run, the inverse relationship works, but in the long run, it seems that the graph will always shift back to a vertical line.