PAL television picture is updated 50 times a second ( NTSC is updated 60). On a standard display, the first 50th (60th) of a second contains lines 1,3,5,7 and so on, the second 50th (60th) the picture is made up of lines 2,4,6,8 etc. This makes up the 25 frames per second (60fps for NTSC). This is known as interlacing and can be seen quite clearly on DVD menu's where the screen flickers slightly - this flickering is more evident on PAL displays because of the lower refresh rate, it also stands out more on text. When you look closely at a television picture you can physically see the displayed lines - they are spaced very slightly. On a larger screen these lines are more apparent. Due to the nature of an NTSC display these gaps are more visible - there are fewer lines of picture information in the same amount of space taken up by the equivalent PAL display. These are the scan lines.