Phase Alternate by line is the 625-line color video system currently used in most of Western Europe, England, Australia, and South Africa.
This is one of several composite video systems. The phase of the color carrier is alternated from line to line. It takes four full pictures for the color to horizontal phase relationship to return to the reference point. This alternation helps cancel out phase errors, the reason the hue control is not needed on PAL TV sets. PAL, in its many forms is used extensively in Western Europe. PAL is often known as "Peace At Last" because of the many compromises made in the evolution of the system and the fact that there are many form of PAL.
The European color television standard that specifies a 25Hz frame rate and 625 lines per frame.