The two fields that constitute a TV frame each have only half the number of lines of a whole frame (576 visible lines for PAL, 480 for NTSC). One field has the odd numbered lines, the other has the evenly numbered ones. One field is first displayed, followed by the other, before the system moves onto a new frame. On the screen, though, the lines of each of these two fields alternate, or are interlaced. Because of the lingering glow of the TV's phosphors and the phenomenon of persistence of vision in our eyes, we see the two fields as one frame.