Ruy Lopez: To Exchange or Not to Exchange

I've started playing the Ruy Lopez, and I know that one of the responses is 3...a6--the  Morphy Defense, I think. I've thought about it, and retreating the bishop 4.Ba4 doesn't make sense to me yet it's one of the most popular continuations.

What are the merits for retreating the Bishop as opposed to entering an exchange variation? Can't Black follow up with 4...b5 and further develop while gaining a tempo and forcing White back? Or is there a refute to 4...b5? Can anyone give me pros and cons of exchanging and of pulling back?

it wrecks blacks queenside pawn structure. making it a softer nut to crack, and black's pieces are still not developed.

if the object of the opening is to develop the pieces and create a strong pawn formation, then black is denied both of these goals. the compensation black get's is space and tempo.

I remember wondering about this same subject when I started playing the Ruy Lopez.

I really liked playing the exchange variation, because it seemed a lot more simple than the great number of possible variations which follow after 1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4.

After 4. Bxc6, an endgame is usually quick to follow (with queens sometimes being traded on d4 or d1), and back then that was to my liking because I thought that I was particularly strong in endgames. White gives black the bishop pair in exchange for a 4-3 kingside majority, while black's own 4-3 queenside majority is crippled by the doubled pawns on the c-file. Black hopes to create counterplay with his bishop pair, and white hopes to get a passed pawn on the kingside and win in the endgame.

Nowadays, I think the general opinion is that the only way for white to retain a real advantage is to avoid the exchange variation. I don't know everything about that, but it's hard to get around the numbers:

White does a little worse in the exchange variation (53.2% vs. 56.6%) but that's not really what's important to look at. It seems hard to avoid a draw in the simplified positions that follow the exchange variation as the draw rate is 45% in those variations and closer to 35% in other variations.

It's a matter of preference, really- but my opinion is that if you want to learn an opening that will allow you to play for a win every time, no matter the opposition, and will follow you as long as you play chess, then learn variations other than the exchange variation.

However, if you consider yourself a particularly strong endgame player and really do not mind trying to convert the tiniest of advantages into a win, the exchange variation is a great weapon to have in your arsenal anyway!

Hope this helped & good luck.

edit: I forgot to address a specific point in your OP.

Black does indeed usually follow up 3...a6 with an eventual ...b5. This usually makes newcomers to the Ruy Lopez wonder what exactly white has achieved with 3. Bb5 except give black the opportunity to expand on the queenside with tempo on the bishop. In the simplest terms, white isn't exactly wasting time with the bishop, as he does get to reroute the bishop to the ideal c2 square. The black pawn advances may also create dark square weaknesses on the queenside & the extended pawns may themselves become eventual targets of attack.

Hmm, this has given me a lot to think about. Much thanks for the feedback.

 Not mentioned yet: since ...a6 and ...b5 are not developing moves themselves (in fact they're weakening the queenside), Black isn't gaining any tempi and isn't developing faster than White is.

don't exchange, play something more interesting. ALthought i like the exchange as black, I usually get a nice fortress for my king with the doubled pawns(always take with the d pawn in the exchange) after castling queenside and a nice kingside expansion with my pawns.