Source : Free On-Line Dictionary of Computing

phase alternating line
     
         (PAL) The {video} signal format used in the UK
        [where else?].
     
        PAL uses {Amplitude Modulation} for the video information, and
        {Frequency Modulation} for the {audio} information.  The phase
        of the {colour subcarrier} is reversed on alternate lines
        which (together with the use of a delay line) allows the
        receiver to cancel any phase errors introduced in the path
        between the studio and the end-user's receiver.  Such phase
        errors are quite common and would cause the displayed colours
        to shift in {hue}.
     
        The US equivalent, {NTSC}, does not have this feature and thus
        requires a user control to correct for transmission phase
        errors, hence the mis-expansion "Never Twice the Same Colour".
     
        (2001-06-08)