If two sine waves are in phase coincidence, the peak amplitude of the resultant wave,which will also be a sine wave, is equal to the sum of the peak amplitudes of the two com-posite waves. The phase of the resultant is the same as that of the composite waves.Phase oppositionWhen two waves begin exactly 1⁄2 cycle, or 180 degrees, apart, they are said to be inphase opposition. This is illustrated by the drawing of Fig. 12-8. In this situation, engi-neers sometimes also say that the waves are out of phase, although this expression is alittle nebulous because it could be taken to mean some phase difference other than 180degrees.If two sine waves have the same amplitude and are in phase opposition, they willexactly cancel each other out. This is because the instantaneous amplitudes of the twowaves are equal and opposite at every moment in time.If two sine waves have different amplitudes and are in phase opposition, the peakvalue of the resultant, which will be a sine wave, is equal to the difference between thepeak values of the two composite waves. The phase of the resultant will be the same asthe phase of the stronger of the two composite waves.The sine wave has the unique property that, if its phase is shifted by 180 degrees,the resultant wave is the same as turning the original wave “upside-down.” Not all wave-forms have this property. Perfect square waves do, but some rectangular and sawtoothwaves don’t, and irregular waveforms almost never do.Leading phaseTwo waves can differ in phase by any amount from 0 degrees (in phase), through 180degrees (phase opposition), to 360 degrees (back in phase again).222 Phase12-7Two sine waves in phase coincidence.