The Nimzo-Indian 4.e3
Ever since Nimzowitsch introduced his flexible, dynamic defence to the queen's pawn, debate has raged over White's best reply. Many variations have been in and out of fashion, but one line in particular has proved an enduring weapon - the sound and solid 4 e3 line, known as the Rubinstein Variation. The 4 e3 Nimzo is extraordinarily rich in strategy. All manner of different central pawn-structures can arise, such as the blocked centre characteristic of the Hübner Variation and a variety of IQP positions. Moreover, in some lines the central tension persists well into the middlegame. The 4 e3 Nimzo provides a stern test of both players' understanding of chess, and so is an ideal choice for those who are looking to win as either colour.
Carsten Hansen is a FIDE Master from Denmark who currently lives in Los Angeles. He has a reputation for writing well-researched, thorough books on major opening systems, and is known to many through his painstaking reviews on the ChessCafe web site.