The technique that allows the end user to extend the raster (the overall area that is scanned by the electron guns) closer to the edge of the monitor's bezel is called overscanning. The opposite occurrence when the display image is focused toward the middle of the monitor is called underscanning.

One must be careful when trying to increase the size of the video image to fill the entire viewable area. The size of the porches, or spacing, between the edge of the raster and video image is a function of the video card's timing. There are very general guidelines and considerations followed by the video card's manufacturer that allow full use of the raster. One drawback is that when placing the video image too close to the edge of the raster, video ringing may be seen. This appears as a thin vertical stripe of lighter or darker intensity with the first one-half inch of the sides of the raster.

To adjust for either occurrence, the end user needs to adjust both the horizontal and vertical sizes and phases. User control of these functions is accomplished using the buttons inside the control panel.

Any arbitrary video signal contains a sizable amount of front and back porch spacing. This spacing depends upon the type of signal, frequency, resolution, and viewing considerations. The monitor manufacturer allows the raster to be overscanned to the edge of the bezel, but the video source also has small porches in order to attain an edge-to-edge display.