Trigger modi of an oscilloscope
If you use the line trigger or the external trigger then the frequency and form of the displayed input channel does not matter. The scope doesn't trigger when the line or external signal are the same as the displayed signal, it triggers when the chosen trigger signal passes through your selected trigger level with your selected slope. If you are looking at a signal that has a different frequency from the line or external signal then the scope will seem to be triggering at random. You should only use these modes when the signal you want to observe is derived from or strongly related to the line or some external signal.