Answer:

It is the system of using birds of different shades of color to produce Standard colored birds.

In order to produce Standard colored exhibition males, a male bird of Standard color is mated to females a few shades darker than Standard. The males from this mating will be good in color and the females very much too dark.

To produce Standard colored exhibition females a male a few shades lighter than the Standard is mated to females that are Standard. The females from this mating will be of the proper color, but the males will be much too light.