With this rather rare choice, Black is trying to put their queen’s bishop on the very same diagonal that White is trying to control with Bg2. Of course, 2. Bg2 Bb7?? is out of the question since White can respond simply with 3. Bxb7, winning a free bishop (and rook!). However, with a move like 2…Nc6, Black can safely fianchetto the bishop. A thematic struggle on the a8-h1 diagonal is to be expected.

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1.g3 b6