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This is a position from a game between Fischer and Ibrahimglu, 1970.

Black's last move was 11. ... Qc7 from d8. According to Stockfish (depth = 28), 12. a4 is the best move for White. The other recommended moves are Nb3 and Rd1. But a4 seems to be better than Nb3 by more than 0.1 point.

It seems to be a very logical move as Fischer played it too. Only the logic behind it is out of my (an intermediate player) reach. :)

The game continued with 12. ... Rad8 13. Nb3 b6 14. Be3 c5... Fischer went on winning this game.

  • A score difference of .1 is surely 'within the noise'. That is, I am not sure any two moves with scores different by .1 are really any different, qualitatively.
    – Tony Ennis
    Dec 29 '15 at 5:35
  • 1
    Yes... the difference is small... but still a4 seems to be a good move at this position, but I would not have considered it here.
    – Srijannath
    Dec 29 '15 at 6:24
  • 1
    Why it's out of reach? a4 is just a standard idea, pushing for queenside spacing and preparing to meet ...Nb6 with a5. The move may also lead to an open a-file for the rook. Very typical idea.
    – SmallChess
    Dec 29 '15 at 8:16
  • I would have been looking at Nb3 followed by developing the black bishop. a4 never would have occurred to me.
    – Tony Ennis
    Dec 29 '15 at 16:10
  • b3 is actually an ineffective square for the knight.
    – limits
    Dec 29 '15 at 17:59

1. a4 allows White to place his knight on the c4-square without having to worry about ...b5 kicking it away because that would be too weakening for Black, because it would make the a-pawn backwards on an open file.

1. a4 also gains space and opens the possibility of a5-a6 to soften the Black queenside for the g2-bishop. If black stops this with ...a6 or ...a5, then Black will have a hole on b6.

A knight on c4 would allow white to move his bishop to f4 to attack the black queen. If Black responds ...e5, this creates a permanent weakness on d6, and White can put his knight there. ...e5 also blocks Black's bishop. The knight on c4 also takes away many squares from the black queen, so if Black does not want to weaken with ...e5, the black queen will have to go to an inactive square.

  • Also, none of the plans you mention for White involve obstructing the open h3-f3 rank, so a4 makes a quick(er) development of the R via h3, another possible plus.
    – Jeff Y
    Dec 29 '15 at 22:13
  • via a3, you mean. Note too that neither of the potential weaknesses of a4 exists here: Black is in no position to mount an attack on a4, and can't quickly occupy b4 with a Knight (the route f6-d5-b4 runs into e4xd5, and even if Black takes the time for Nd7-b8-a6-b4 White can comfortably eject the steed with c3). Dec 31 '15 at 1:13
  • With the game continuing 13. Nb3, this would seem incorrect, since White never places his knight on c4.
    – Allure
    Jan 12 '20 at 23:36

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