Moiré  The Moiré effect consists of arched distortion lines over the screen which change when picture width and height are changing. Moiré occurs only at certain resolutions. The interference is caused by interactions between the video signal and the shadow mask/CRT pitch. They appear mainly at line or point patterns. Unfortunately extreme Moiré patterns occur when the monitor has a very good focus. Good test patterns are line and point patterns which are displayed over the whole screen. Moiré patterns are often most visible if a screen background is set to a pattern of dots or lines.