He was one of the very few modern grandmasters to have employed Bird's Opening (1.f4) with any regularity, and had a long-term association with the move 1.b3, a system commonly known as Larsen's Opening or the Nimzo–Larsen Attack in his (and Aron Nimzowitsch's) honor. He played the Dutch Defence with success at a time when the opening was rarely seen at the top level. He revived the almost dormant Bishop's Opening (1.e4 e5 2.Bc4) with success in 1964 and explored new ways for Black to seek activity in the Philidor Defence (1.e4 e5 2.Nf3 d6). Indeed, he wrote a short monograph on the Philidor, "Why Not The Philidor Defense?", in 1971. He was also the first top player to successfully use the Grand Prix Attack against the Sicilian Defence (1.e4 c5 2.f4), spurring a sudden and sustained gain in its popularity. In the mid-1960s, he showed surprising faith in Alekhine's Defence (1.e4 Nf6) and even employed it on important occasions. He played the rare Scandinavian Defence 1.e4 d5 to defeat World Champion Anatoly Karpov in 1979, sparking renewed interest in that variation. A favourite line in the Caro–Kann Defence (1.e4 c6 2.d4 d5 3.Nc3 dxe4 4.Nxe4 Nf6 5.Nxf6+ gxf6) is co-named for him and David Bronstein; the idea is to accept a weakness to the Black pawn structure in exchange for an unbalancing of the position and retaining the bishop pair.