Chess Board
[ECO code D31] Move Move Move Move Here Move Here Move Here
SEMI-SLAV DEFENCE
Main line (alt. route)

White's KKt supported a planned K4(e4) pawn.
Black usually deploys his King's Knight to KB3(f6) to quadruple support for his QP and deter White's ongoing P-K4 plan, although he invites a 5.B-Kt5 pin. "Main" to the position until '60s ECO coding. B-Alt.
     White   Black
 1.  P-Q4    P-Q4
 2.  P-QB4   P-K3
 3.  Kt-QB3  P-QB3
 4.  Kt-B3   Kt-B3  Main 4B,3B route

WHITE to Move or Undo or Jump or Clear
 
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