The Timebase
The time taken for the spot to sweep across the screen horizontally is set
by the timebase generator - more usually known simply as 'the timebase'.
The more quickly the spot moves across, the fewer vertical cycles will be displayed.
As with the Y-Amplifier, different values can be set with a panel-mounted switch but, in the case of the timebase, it is marked in units of time/division.
The switch shown in the picture is set to 10mS/division. One millisecond is a 1000th of a second so, in the bottom picture, 20 mS (ie 2 hundredths of a second) elapse between consecutive positive peaks of the waveform.
Note the horizontal position adjuster at the top-left of the panel to enable the waveform to be aligned with the vertical gridlines on the screen.