411 REFERENCE · Wikibooks
Chess Opening Theory/1. b4/1...e6/2. Bb2/2...Nf6/3. b5/3...a6/4. a4/4...axb5
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Polish Opening: Queenside Defence
4...axb5
Black trades pawns on b5. White should recapture with the obvious /5. axb5/, as their rook on a1 is defended by the bishop on b2 so Rxa1 would be a trade.