This figure illustrates variations in a just interval that occur when a time delay is introduced between the two sinusoidals.  We begin with the perfect fifth shown in top portion of the figure.  The periodicity, Tx{\displaystyle T_{x}} holds only to the extent that the interval is a perfect just ratio of two integers.  Before considering small tempered variations from perfect justness, we first address the fact that a just interval is defined not only by the frequency ratio, but also by any phase difference that might exist between the two sinusoidals that.  A perfect fifth is first shown with zero phase shift, assuming that we adopt adopt the convention that both signals are sine waves (the phase shift is π/4{\displaystyle \pi /4} if they are cosines.) The reader can visually verify that advancing the low frequency signal by time, t=T0{\displaystyle t=T_{0}} returns this phase shift back to zero, for both the fifth, as well as the tritone.  To facilitate visualization of this effect, alternate cycles of the sine wave are shaded (blue or yellow.)  The region of unshifted sine waves can be recognized by the fact that only the corners of the two shaded regions are in contact with each other (at the start of a new period of length Tx{\displaystyle T_{x}}.)
Own work
Guy vandegrift