LEADING PHASE

Suppose that there are two sine waves, wave X and wave Y, with identical frequencies. If wave X begins a fraction of a cycle earlier than wave Y, then wave X is said to be leading wave Y in phase. For this to be true, X must begin its cycle less than 180° before Y. Figure 1 shows wave X leading wave Y by 90°. The difference can be anything greater than 0°, up to but not including 180°.
Leading phase is sometimes expressed as a phase angle Φ such that 0° < Φ < +180°. In radians, this is 0 < Φ < +π. If we say that wave X has a phase of +π/2 rad relative to wave Y, we mean that wave X leads wave Y by π/2 rad.

Fig. 1. Wave X leads wave Y by 90°.