Lagging phaseSuppose that wave X begins its cycle more than 180 degrees, but less than 360 degrees,ahead of wave Y. In this situation, it is easier to imagine that wave X starts its cycle laterthan wave Y, by some value between 0 and 180 degrees. Then wave X is not leading, butinstead is lagging, wave Y. Figure 12-10 shows wave X lagging wave Y by 90 degrees.The difference could be anything between 0 and 180 degrees.224 Phase12-10Wave X lags wave Y by 90 degrees.You can surmise by now that leading phase and lagging phase are different ways oflooking at similar “animals.” In practice, ac sine waves are oscillating rapidly, sometimesthousands, millions, or even billions of times per second. If two waves have the samefrequency and different phase, how do you know that one wave is really leading theother by some small part of a cycle, instead of lagging by a cycle and a fraction, or by afew hundred, thousand, million, or billion cycles and a fraction? The answer lies in thereal-life effects of the waves. Engineers and technicians think of phase differences, forsine waves having the same frequency, as always being between 0 and 180 degrees, ei-ther leading or lagging. It rarely matters, in practice, whether one wave started a fewseconds earlier or later than the other.So, while you might think that the diagram of Fig. 12-9 shows wave X lagging waveY by 270 degrees, or that the diagram of Fig. 12-10 shows wave X leading wave Y by 270degrees, you would get an odd look from an engineer if you said so aloud. And if you saidsomething like “This wave is leading that one by 630 degrees,” you might actually belaughed at.Note that if wave X (the dotted line in Fig. 12-10) is lagging wave Y (the solid line),then wave X is somewhat to the right of wave Y.