The screen on LCD and Plasma displays is made up of a fixed number of thousands of tiny pixels.A 15" flat-screen with a resolution of 1024 x 768 for example, has 786423 pixels. Each pixel comprises three sub-pixels (red, green and blue) so on the 15" above, there's almost 2.4 million dots in total. Behind each pixel is a transistor. When a transistor fails, it manifests itself as a bright, dark or coloured dot on the screen that stands out from the rest.