When the beam reaches the right side of the bottom line, it has to move back to the upper left corner of the screen, as represented by the green line in the figure. The term horizontal retrace is used to refer to the beam moving back to the left at the end of each line, while the term vertical retrace refers to its movement from bottom to top. As the beam paints each line from left to right, the intensity of the beam is changed to create different shades of black, gray and white across the screen. Because the lines are spaced very closely together, your brain integrates them into a single image. A TV screen normally has 525 lines visible from top to bottom.
All TVs use an interlacing technique when painting the screen.
In this technique, the screen is painted 60 times per second but only half the lines are painted per frame. The beam paints every other line as it moves down the screen, for example every odd-numbered line. The entire screen, in two passes, is painted 30 times every second. The alternative to interlacing is called progressive scanning , which paints every line on the screen 60 times a second. Most computer monitors use progressive scanning because it significantly reduces flicker.