LAGGING PHASE

Suppose that wave X begins its cycle more than 180° but less than 360° ahead of wave Y. In this situation, it is easier to imagine that wave X starts its cycle later than wave Y by some value between but not including 0° and 180°. Then wave X is lagging wave Y. Figure 1 shows wave X lagging wave Y by 90°. The difference can be anything between but not including 0° and 180°.
Lagging phase is sometimes expressed as a negative angle Φ such that -180° < Φ < 0°. In radians, this is -π < Φ < 0. If we say that wave X has a phase of -45° relative to wave Y, we mean that wave X lags wave Y by 45°.

Fig. 1. Wave X lags wave Y by 90°.