7

1...c5 initiates the Sicilian Defense. It is absolutely a respectable opening, and I am surprised you've never seen it before. I'll just quote Wikipedia: The Sicilian is the most popular and best-scoring response to White's first move 1.e4. Opening 1.d4 is a statistically more successful opening for White because of the high success rate of the Sicilian ...


2

1...c5 is a great move that fights for the center even better perhaps than 1...e5. By playing ...c5, White will only be able to play d4 either by trading one of his central pawns for a side pawn or by preparing it with c3 (a move the b1 knight won't be happy about). On the other hand, 1...c5 is not a developping move (and does not help development at all), ...


2

Any answer to the question will be as dangerously biased as the database behind it, and moreover Stockfish will have other opening "reasons" than a human (I play 100% c5, and that even without knowing theory...), but here are some values from the database behind the Lichess analysis board, as number of games and white win-white loss: c5 - Sicilian ...


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