Major Line

In the fight for three adjacent key squares only taking the opposition on the major line brings success. When the enemy king retreats from the major line you must outflank him.
How do we determine the major line? We must use our abstract reasoning skills and imagine a fantasy position. For example, from the starting position of this endgame

we might imagine a fantasy position such as this

or this

from where we are assured that the opposition will be successful. Since we are taking the opposition on the 7th rank, therefore the 7th rank is the major line. In other words, White wants to take the opposition on the 7th rank and outflank on the other ranks.