Apart from single-shot measurements, the signal that is going to be measured must be repetitive. Signals a, b, and c in Fig. 1.9 are all repetitive because a span of time can be defined so that the same signal is repeated sequentially.
Very often hum is present on the signals under test. This can be easily determined from the screen because the hum is related to the line frequency. If a signal shows a kind of unexpected amplitude modulation, switching back the time-base setting to about 5 to 10 or 20 ms/div, and switching over the trigger source selector to MAINS (or LINE), will generally result in a stable picture in the event of hum.