Phase coincidence:

When two sine waves have the same frequency, they behave differently if their cycles begin at different times. Whether or not the phase difference, called as phase angle and specified in degrees, matters which depends on nature of the circuit.

Phase angle can have meaning only when two waves have identical frequencies. If the frequencies vary, even by a little bit, relative phase which constantly changes, and you can not specify the single number. In the discussions of phase angle, assume that the 2 waves have identical frequencies.

Phase coincidence means that two waves begin at exactly the same moment. They are lined up. This is shown in Figure for 2 waves having different amplitudes. The phase difference in this case is 0 degrees. You may say it is any multiple of 360 degrees, too, but engineers and technicians almost never speak of any phase angle of less than 0 or over 360 °.

Figure--   Two sine waves in the phase coincidence.

If 2 sine waves are in the phase coincidence, the peak amplitude of resultant wave, which will be a sine wave, is equal to the sum of the peak amplitudes of the 2 composite waves. The phase of resultant is the same as that of composite waves.