For an image to remain stable, the phosphors must be reactivated by repeated scans of the electron beam. After the beams make one horizontal sweep across the screen, the electron streams are turned off as the electron guns refocus the paths of the beams back to the left edge of the screen at a point just below the previous scan line. Called raster scanning . Upon completing a screen, the beams return to the upper-left corner to begin a new screen. The screen is normally redrawn, or refreshed, about 60-70 times a second.
Some video cards scan only every other line when they scan over a screen.
Called interlacing . But also causes flicker. Don't want to buy this type of monitor.