Evans Gambit Accepted

	a	b	c	d	e	f	g	h
8									8
7									7
6									6
5									5
4									4
3									3
2									2
1									1
	a	b	c	d	e	f	g	h

5. c3

With 5. c3, White attacks the bishop and also supports the d4 advance in the future.

Black must do something about the bishop, and there are 5 safe squares for the bishop.

5...Bf8?! is the Mayet defence. This undevelops the bishop, and is the worst move of the five, as in order to castle kingside, Black must move it again in addition to the g8-knight. White almost always play 6. d4 in order to punish this move, in which he has gained a comfortable advantage.

5...Bc5?! is the McDonnell Defence. This is usually only played by beginners, thinking this move stops the d4 push, but it actually doesn't, and white can then play 6. d4 which also gains a tempo on the bishop.

5...Bd6!? is the Stone-Ware Defence. Although this blocks the d-pawn, it also adds a defender to the e5-pawn, so when 6. d4 happens, it won't come with an attack on the pawn.

5...Be7 is the Anderssen variation. This is arguably the safest square for the bishop, as it won't be attacked again in the near future, and also protects the g5 square, not allowing a white knight there. In addition, it allows the c6-knight to come to a5 and attack to the bishop, which is not possible in the main line with 5...Ba5. After white's automatic response of 6. d4, Black responds with 6...Na5, which drives the bishop away from the powerful diagonal.

5...Ba5 is the main line. This keeps the bishop on the a5-e1 diagonal, and therefore pins the c3 pawn when 6. d4 occurs, so that when 6...exd4 (Pierce Defence) is played, White can't simply recapture the pawn. However White gains immense attacking chances after either 7. Qb3 or 7. O-O, so Black more often plays 6...d6 (Bronstein Defence), in which he's achieved a solid position, though White still has good attacking chances.

. 1. e4 e5 2. Nf3 Nc6 3. Bc4 Bc5 4. b4 Bxb4 5. c3

Wikipedia has related information at Evans Gambit.

• Kasparov, Garry, & Keene, Raymond 1989 Batsford chess openings 2. ISBN 0-8050-3409-9.