In the late 1960s the stable negatively sloped Phillips curve was overturned by the Friedman–Phelps natural rate model. Their Phillips curve was vertical in the long run at the natural unemployment rate, and their short-run curve shifted up whenever unemployment was pushed below the natural rate. This paper criticizes the underlying assumption of the Friedman–Phelps approach that the labor market continuously clears and that changes in unemployment down or up occur only in response to ‘fooling’ of workers, firms, or both. A preferable and resolutely Keynesian approach explains quantity rationing by inertia in price and wage setting. The positive correlation of inflation and unemployment in the 1970s and again in the 1990s is explained by joining the negatively sloped Phillips curve with a positively sloped dynamic demand curve. For any given growth of nominal GDP, higher inflation implies slower real GDP growth and higher unemployment. This ‘triangle’ model based on demand, supply, and inertia worked well to explain why inflation and unemployment were both positively and negatively correlated between the 1960s and 1990s, but in the past decade the slope of the short-run Phillips curve has flattened as inflation exhibited a muted response to high unemployment in 2009–2013 and low unemployment in 2016–2018.