A rarely played move against the Queen's gambit, which has similarities with whites London-system. This move develops the light-squared bishop to control the e4 square with a piece and therefore wants to deny the e4 push from white. This declines the gambit in a way, where black can't reinforce his pawn on d5 and isn't able to control the e4-square with a pawn after cxd5.

This is less common as white has no problems to seize an advantage in the game.

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1. d4 d5 2. c4 Bf5

Wikipedia has related information at Baltic Defence.

• Kasparov, Garry, & Keene, Raymond 1989 Batsford chess openings 2. ISBN 0-8050-3409-9.