The Mason Variation arises after 1.d4 d5 2.Bf4. ECO code: D00.

Named after James Mason (1849–1905). This is the London System—one of the most popular setups at club level. White develops the bishop before Nf3 (which would block the bishop's path to f4). Black responds with ...Nf6, ...c5 or ...e6 and natural development. The system is solid and reliable for White.