We shall now go back to the endings in search of a few more principles, then again to the middle-game, and finally to the openings once more, so that the advance may not only be gradual but homogeneous. In this way the foundation on which we expect to build the structure will be firm and solid.
In the position shown below, White can draw by playing b4 according to the general rule that governs such cases, i.e. to advance the Pawn that is free from opposition. But suppose that White, either because he does not know this principle or because he does not, in this case, sufficiently appreciate the value of its application; suppose, we say, that he plays 1. a4. Then Black can win by playing 1... a5, applying one of the cardinal principles of the high strategy of chess -- A unit that holds two.
In this case one Pawn would hold two of the opponent's Pawns. The student cannot lay too much stress on this principle. It can be applied in many ways, and it constitutes one of the principal weapons in the hands of a master.
This brings the game to a position which is won by Black, and which constitutes one of the classical endings of King and Pawns. I shall try to explain the guiding idea of it to those not familiar with it.