Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...f5/4. d3/4...fxe4/5. dxe4/5...Nf6/6. O-O

Ruy Lopez, Schliemann Defence

	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

Common moves:
6...Bc5 - Open Variation
6...d6 - Closed Variation

Time for Black to develop some more pieces. Spoiler: the pawn currently on d7 isn't going to make it to d5 at this point, because that would leave e5 hanging. So it'll have to go to d6 if the c8-bishop wants to get out.

The immediate 6...d6 would shut in the f8-bishop. That's okay, it's got a perfectly good square on e7.
6...Bc5 makes sure that the bishop doesn't get shut in, but becomes a pawn sacrifice after 7. Bxc6 bxc6 8. Nxe5.

If ...Bc5 is often a luxury Black can't afford in the Ruy Lopez, surely this applies double in the Schliemann Defence where the Black kingside has already been opened up with ...f5? Black's argument is that firstly, the open f-file is an attacking strength as well as a defensive weakness, and allows Black to fantasize about targeting f2 with ...O-O and ...Nxe4/...Ng4 to go with ...Bc5, so that gives White something to worry about. Secondly, the fact that the d-pawn is gone means that White doesn't have the simple plan of c3 and d4 with gain of time against the bishop.

If Black captures on e4 immediately, White has numerous strong options. 7.Nxe5, threatening 8. Qh5+, has to be met with 7...Nxe5, and then 8.Re1 wins the piece straight back with an ongoing attack (8...Nf6? 9.f4!) The immediate 7. Re1 or Qe2 are also strong, as 7... d5 allows 8.Nxe5. Regardless, the e file becomes a huge liability for Black with the king still stuck in the center.