Restricting the enemy bishop and parrying the threat of 3 b5.

In contrast, 2...axb4? 3.axb4 is illogical since it opens the a-file for the white rooks.

The b2-bishop is no longer impressive, while the c4-pawn has turned into a weakness, something that Black soon exploits.

Followed by ...Rad8, leads to a clear black advantage – Keres.

A simple but strong restrictive measure. Other moves, such as 1.Bf4?! permit 1...Ba6! , perhaps followed by ...Bd3-g6, and Black drastically improves his worst piece.

1...Qd7 , preparing ...Bc6-b5, then White should consider 2.g4 Bc6 3.g5 Bb5 4.Qf3 , intending g2-g3, Kg2 and Rh1 with attacking chances.

A good diagonal for the bishop, where it prevents a liberating ...e6-e5 advance.

Swapping Black's "good" bishop in order to exploit his dark square weaknesses more efficiently.

White is clearly better.