Leading phase

Two waves can differ in phase by any amount from 0 degrees (in phase), through 180 degrees (phase opposition), to 360 degrees (back in phase again).

Suppose there are two sine waves, wave X and wave Y, with identical frequency. 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 degrees before Y. Figure 1 shows wave X leading wave Y by 90 degrees of phase. The difference could be anything greater than 0 degrees, up to 180 degrees.

Figure 1: Wave X leads wave Y by 90 degrees.

Note that if wave X (the dotted line in Fig. 1) is leading wave Y(the solid line), then wave X is somewhat to the left of wave Y. In a time line, the left is earlier and the right is later.