Chess Board
[ECO code C23]
(CLASSICAL) BISHOP'S OPG. - Philidor Var.
Side Variation

White's QBP exchanges, removing Black's newly doubled QP on Q4(d4), and attacks Black's bishop.
Black's Queen on K2(e2) removes White's KP with check, for 6.Q-K2..QxQ, 7.KtxQ..B-Kt3 gaining a pawn.
	White	Black
 1.	P-K4	P-K4
 2.	B-B4	B-B4
 3.	P-QB3	Q-K2!?
 4.	P-Q4?!  PxP
 5.	PxP	QxP ch

Example ends: Undo or Jump or Clear