 When the overall interlaced framerate is 60 frames per second, a pixel (or more critically for e.g.windowing systems or underlined text, a horizontal line) that spans only one scanline in height is visible for the 1/60 of a second that would be expected of a 60 Hz progressive display - but is then followed by 1/60 of a second of darkness (whilst the opposite field is scanned), reducing the per-line/per-pixel refresh rate to 30 frames per second with quite obvious flicker.To avoid this, standard interlaced television sets typically do not display sharp detail.