When using Lissajous figures to determine phase differences, an indication of zero or 180 degrees is represented on the screen of an oscilloscope by:

Lissajous figures are generated on an oscilloscope in X-Y mode, where one signal is applied to the horizontal input and another to the vertical input. When the two signals have the exact same frequency but differ in phase by zero or 180 degrees, the resulting trace is a diagonal straight line. If the phase difference is exactly zero degrees, the signals rise and fall together, creating a diagonal line from bottom-left to top-right. If the phase difference is exactly 180 degrees, one signal rises while the other falls, producing a diagonal line from top-left to bottom-right. A circle or an ellipse would only appear if there is a phase difference other than zero or 180 degrees, such as 90 degrees for a perfect circle when the amplitudes are equal.