The theory of corresponding squares is the ideal tool for seeking solutions in blockaded positions where there are at least two possible ways of a breakthrough into the enemy position. This is nicely illustrated by a C. Locock’s study published as early as 1892. We should start looking for corresponding squares beginning with the critical squares from which White can bring about the breakthrough into the enemy position. Which situations are obviously critical? The graphic tabulation of corresponding squares will be as in diagram P-223.