The refresh rate is the rate at which the display is updated. For field-based systems, this is the rate at which fields are displayed, and for frame-based systems it is the rate at which the frames are displayed. These two methods are called interlaced scanning and progressive scanning, respectively. Interlaced scanning updates the odd or even lines for a field once per refresh, whereas progressive scanning updates all the scanlines once per refresh. Hence video systems use interlaced scanning, whereas computer graphics systems use progressive scanning10.