Not synchronised - Animated GIF      positive trigger     negative trigger
In order to display a steady pattern on the screen, the time of the sweep across the screen must be related to one cycle of the waveform and the sweep must start at some definite point of each cycle. To ensure this, the timebase is synchronised or triggered by the waveform. The animation on the left shows the effect of having this trigger level incorrectly set.

In the centre picture, the timebase is triggered when the waveformn reaches a certain positive value and, in the third picture, the timebase is triggered when the waveform reaches a certain negative value. In each case, the display is perfectly stationary. Note that the number of mS/division remains the same - only the moment at which the sweep starts across the screen changes.