White’s King’s Bishop is not well placed either and is practically condemned to play the role of a Pawn by protecting the square d3 without, at the same time, attacking anything.  There is, of course, a chance for him to be useful in the diagonal a2-g8.  It may be said that in King’s Pawn openings White’s King’s Bishop comparatively seldom has an opportunity to take an active part in the battle.  He is mostly exchanged at an early stage of the game for Black’s Queen’s Knight or Queen’s Bishop.  In Queen’s Pawn openings, however, he finds a great field of action in the unobstructed diagonal b1-h7.  In this diagonal he can also be used in all openings starting with P-e4 in which Black does not advance his King’s pawn to e5, as White can open the diagonal at any time by playing P-e5.

Generally speaking, Bishops should not be placed in diagonals which are obstructed by Pawns of their own army, and Pawns moves should be avoided which close a diagonal formerly open to a kindred Bishop.  A striking illustration of the importance of this rule will be found in the play which developed in the position of Diagram 55 in a game between Teichmann and Dus Chotimirski in the Prague Tournament 1908.  Black, on the move, played (1)..., Kt-e5, disturbing the symmetry of the position to his advantage by opening the diagonal of his Queen’s Bishop without allowing White to make a similar maneuver.  After (2) Ktxe5, Bxe5; (3) Q-e2, o-o; (4) Ra1-d1, Q-e7; White yielded the temptation to drive away the disagreeable Bishop e5 by (5) P-f4, a move which had to be followed up with the advance of the e-Pawn who otherwise would have remained very weak, not being protected by another Pawn.

+---------------------------------------+
8 | #R |    |    | #Q | #K |    |    | #R |
|---------------------------------------|
7 |    | #B |    |    |    | #P | #P | #P |
|---------------------------------------|
6 | #P |    | #Kt| #B | #P | #Kt|    |    |
|---------------------------------------|
5 |    | #P |    |    |    |    |    |    |
|---------------------------------------|
4 |    | ^P |    |    |    |    |    |    |
|---------------------------------------|
3 | ^P |    | ^Kt| ^B | ^P | ^Kt|    |    |
|---------------------------------------|
2 |    | ^B |    |    |    | ^P | ^P | ^P |
|---------------------------------------|
1 | ^R |    |    | ^Q |    | ^R | ^K |    |
+---------------------------------------+
a    b    c    d    e    f    g    h

Diagram 55.

The game went on as follows:  (5) ..., B-c7; (6) P-e4, B-b6+; (7) K-h1, Rf8-d8; (8) B-b1, Ra8-c8; (9) Rxd8+, Qxd8; (10) R-d1, Q-e7; (11) P-e5.  This opens again the diagonal of the King’s Bishop, but it closes that of the Queen’s Bishop, and it is the advantage of the work done by his Queen’s Bishop in the unobstructed diagonal which secures Black the victory, (11) ..., Kt-d5; (12) Ktxd5, Bxd5; (13) Q-g4, Q-b7; (14) P-f5, R-c4; (15) Q-g3, R-f4!.  White cannot capture the Rook on account of Bxg2 mate. (16) P-f6, P-g6.  There is now no defense against R-f2 which attacks g2 and b2 at the same time. (17) B-a2, R-f2; (18) Bxd5, Qxd5! and White resigns as he loses his Bishop on account of the mating threat.