In similar positions there are two impenetrable positions which must be known in the very beginning. Both are based on preventing the breakthrough of the white rim pawn onto the fifth rank! Let us first examine the bishop defence on the shorter defence diagonal, which is possible when the white g-pawn had stepped on the fourth rank, in front of the rim pawn. The following position was found in the Olafsson – Larsen game (Las Palmas, 1974) . White to move cannot win since he cannot break the blockade along black squares and realize h4-h5.
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