The principle of two weaknesses

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Eric Rosen-Marissa Li, City of Chicago Class 2017 (Black to play)

When a player's position has only one weakness (an isolated pawn or a chronically weak square), a skillful defender can often take precautionary measures. But when one's postions has two weaknesses, the attacker can alternate between attacking the two weaknesses, or even try to create a third.

International Master Eric Rosen was kind enough to send us annotations to his first-round game against Marissa Li, which nicely illustrates this annoyingly effective strategy.