A dual-scan display, which is sometimes referred to as a passive-matrix display, has a row of transistors running on the top of the screen (x-axis) as well as the left of the screen (y-axis). The amount of transistors are defined by the LCD manufacturer, for example, the manufacturer may define 800 transistors along the x-axis and 600 transistors along the y-axis. These transistors power the pixels that are powered in grids by the transistors. Unfortunately, if one of these transistors were to fail, you would receive a solid black line going vertical or horizontal.
When compared to an active-matrix, most passive-matrix is superior technology. With passive-matrix, you cannot see the screen unless you are directly in front of it. However, the display is usually dimmer than a passive-matrix display.
Newer passive-matrix technologies include CSTN (color super-twisted nematic), DSTN (double layer super-twist nematic), and HPA (high-performance addressing) displays. They offer higher contrast and brightness than older passive-matrix displays.
In conclusion, the passive-matrix today, while better than previous versions, is still an affordable solution as an LCD. However, it does not offer the quality that is found with active-matrix.