Many less expensive consumer displays that bear “HD Ready” label are incapable of displaying the full 1920×1080 raster size of the largest HD standard. They internally downconvert larger image to the maximum pixel resolution they can display. On such monitors it may be difficult to discern any visible difference between 1080 and 720 size signals.
Another set of numbers commonly seen on HD monitors is contrast ratio. This ratio describes the maximum dynamic range between darkest black and the brightest white.