What a trigger does

A repeating signal looks the same every cycle, but each capture starts at a random moment. Drawn one after another, the captures would jump left and right.

A trigger fixes this. It looks for the moment the signal crosses a chosen voltage in a chosen direction, and lines up every capture so that moment is at the same place on the screen. The trace then holds still.

The same idea catches single events: tell the scope to wait for the crossing and keep only what happens around it.

See the Trigger controls.