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Hermann Keidanski (later Keidanz and Kaidanz; born November 4, 1865, Władysławowo (now Poland) - December 1938, Germany) was a Jewish Polish-German chess master.
Born in Pomerania (Pomorze, Pommern), he had come to Berlin where participated in many chess tournaments in the 1890s. He tied for 8-9th at Cologne 1898 (the 11th DSB Congress, Hauptturnier A, Ottokar Pavelka won), took 2nd, behind Julius Finn, at New York 1903, and tied for 5-6th at New York State Chess Association 1907.
He lost a match to Carl August Walbrodt (1-5) in 1891, and won against Eugene Delmar (4-1) in 1902.
His name is attached to the Keidanski Variation in the Prussian Defence (1.e4 e5 2.Nf3 Nc6 3.Bc4 Nf6 4.d4 exd4 5.e5 d5 6.Bb5 Ne4 7.Nxd4 Bc5 8.Nxc6!? Bxf2+ 9.Kf1 Qh4!). Analysis by Dr. Hermann Kaidanz appeared in the Wiener Schachzeitung in 1904. He also analysed the Keidansky Gambit (1.e4 e5 2.Bc4 Nf6 3.d4 exd4 4.Nf3 Nxe4 5.Qxd4).