By contrast, the late 1990s episode of below-4-percent unemployment was quite brief, and during the episode and surrounding quarters inflation was reasonably stable and remained below 2 percent. To explore the Phillips curve relationship in these two periods more closely, we need to bring in the concept of the natural rate of unemployment.
In standard economic thinking, an unemployment rate above the natural rate indicates slack in the labor market and tends to be associated with downward pressure on inflation; unemployment below  -5the natural rate represents a tight labor market and is associated with upward inflation pressure.