user10238: I've recently begun studying openings, and I've been practicing the Ruy Lopez.
I played a game that began as follows.
[FEN ""]

1. e4 e5 2. Nf3 Nc6 3. Bb5 d5 4. O-O dxe4 5. Nxe5 Qd5 6. Bxc6+ bxc6 7. Ng4

Generally the black response is 3... a6, and then I can either play the exchange variation or back up my bishop to Ba4. Instead, as seen, they played 3... d5. I responded by ignoring the threat to my pawn by castling. After 5... Qd5, I decided to make the exchange and backed up my knight.
Did I respond correctly? Is there a named variation for that black response? If anything it was enough to throw me off. A few moves later I traded queens and forked his king and rook, but I can't tell if my advantage came from a bad move by my opponent or as a result of his atypical opening leaving him in a disadvantaged situation.

user657: ...d5 is seen in a number of variations, though not this early. Probably the earliest is in a line from Cordel’s that GM Konikowski concocted in some correspondence games (3. ... Bc5 4. c3 d5). Grab them from one of the online databases; some of them are wonderfully complex studies of the kind of pitfalls White can stumble into.
Typically, when it shows up in the early phases of the opening, the idea for Black is some enterprising piece play that can catch White unawares if he’s not careful. Typical ideas involve a Queen sortie to either f6 or g5, getting the Queen off the hot seat at d8, often with gain of tempo (from g5 she eyes g2, and from f6 she protects the e-pawn as well as defends c6). Paired with the now freed Bf8, and the light squared B on d6 or c5, White needs to be careful about trying to hard to overrun Black. The lines are highly tactical, and probably not all that sound, but if Black is sharp enough, White can often get lost in the complexities of the position. There are enough resources in the position for a good coffeehouse player to cause a great deal of trouble.
My first impression is this is too early to be really dangerous, so if you feel sharp enough, or Black is clumsy, you should be able to enter in to the melee and come out ahead. If not, straightforward development will see White through to an advantage. (That’s the main problem with attempts like this to wrest the initiative from White; White can typically ignore the bait, make solid moves, and still come out with the better position.

user10087: The variation proposed is interesting and the one favoured by the computer. I went further and thought when seeing your variation (d5) that it reminded me of certain variations of the Ponziani, with one single (huge) difference: white has not yet played c3. So, taking on d5 to implement Nc3 came to my mind, with the following variation (as a sample and not definitively concluding - and with computer assistance): 
[FEN ""]

1.e4 e5 2. Nf3 Nc6 3. Bb5 d5 4. exd5 Qxd5 5. Nc3 Qd6 {for instance} 6. Qe2 Nge7 7. Nxe5 Be6 8. f4 0-0-0 9. Ne4 Qd4 10. c3 Qb6 11. Nc4

etc. With an advantage to white. Of course, the line suggested by Tony Ennis is less "messy" and more clear-cut but I suggest another option. It is up to you to make your choice.  The variations we both propose are different in nature. Tony Ennis suggests you go for a material advantage in exchange for the bishop pair (for what it is worth without convenient development). My variation also proposes a material advantage but doesn't surrender the bishop pair. Furthermore, my variation keeps the center solid (pawns in the center), avoiding counterplay and hindering free piece play on black's part. 
The idea of d5 is thematic, of course, but premature and clearly inferior but it has one advantage: you are on your own and one single misstep and problems appear. It could be interesting in blitz play but not more, i think. The positions arising after d5 should be examined by you (and possibly others) with computer assistance to refute your missteps in analysis and to get an idea of how to handle various responses. 

user70: 3... d5 is not listed here so it is a safe bet there's a problem.
It looks like an overreach, with both Black pawns being vulnerable.
I'd go for a pawn steal.


[FEN ""]

1. e4 e5 2. Nf3 Nc6 3. Bb5 d5
4.Nxe5 a6 5.Bxc6+ bxc6 6.Qe2 {White has better development, better pawns, and a free pawn.}



10238: <p>I've recently begun studying openings, and I've been practicing the Ruy Lopez.</p>

<p>I played a game that began as follows.</p>

<pre><code>[FEN ""]

1. e4 e5 2. Nf3 Nc6 3. Bb5 d5 4. O-O dxe4 5. Nxe5 Qd5 6. Bxc6+ bxc6 7. Ng4
</code></pre>

<p>Generally the black response is 3... a6, and then I can either play the exchange variation or back up my bishop to Ba4. Instead, as seen, they played 3... d5. I responded by ignoring the threat to my pawn by castling. After 5... Qd5, I decided to make the exchange and backed up my knight.</p>

<p>Did I respond correctly? Is there a named variation for that black response? If anything it was enough to throw me off. A few moves later I traded queens and forked his king and rook, but I can't tell if my advantage came from a bad move by my opponent or as a result of his atypical opening leaving him in a disadvantaged situation.</p>
 657: <p>...d5 is seen in a number of variations, though not this early. Probably the earliest is in a line from Cordel’s that GM Konikowski concocted in some correspondence games (3. ... Bc5 4. c3 d5). Grab them from one of the online databases; some of them are wonderfully complex studies of the kind of pitfalls White can stumble into.</p>

<p>Typically, when it shows up in the early phases of the opening, the idea for Black is some enterprising piece play that can catch White unawares if he’s not careful. Typical ideas involve a Queen sortie to either f6 or g5, getting the Queen off the hot seat at d8, often with gain of tempo (from g5 she eyes g2, and from f6 she protects the e-pawn as well as defends c6). Paired with the now freed Bf8, and the light squared B on d6 or c5, White needs to be careful about trying to hard to overrun Black. The lines are highly tactical, and probably not all that sound, but if Black is sharp enough, White can often get lost in the complexities of the position. There are enough resources in the position for a good coffeehouse player to cause a great deal of trouble.</p>

<p>My first impression is this is too early to be really dangerous, so if you feel sharp enough, or Black is clumsy, you should be able to enter in to the melee and come out ahead. If not, straightforward development will see White through to an advantage. (That’s the main problem with attempts like this to wrest the initiative from White; White can typically ignore the bait, make solid moves, and still come out with the better position.</p>
 10087: <p>The variation proposed is interesting and the one favoured by the computer. I went further and thought when seeing your variation (d5) that it reminded me of certain variations of the Ponziani, with one single (huge) difference: white has not yet played c3. So, taking on d5 to implement Nc3 came to my mind, with the following variation (as a sample and not definitively concluding - and with computer assistance): </p>

<pre><code>[FEN ""]

1.e4 e5 2. Nf3 Nc6 3. Bb5 d5 4. exd5 Qxd5 5. Nc3 Qd6 {for instance} 6. Qe2 Nge7 7. Nxe5 Be6 8. f4 0-0-0 9. Ne4 Qd4 10. c3 Qb6 11. Nc4
</code></pre>

<p>etc. With an advantage to white. Of course, the line suggested by Tony Ennis is less "messy" and more clear-cut but I suggest another option. It is up to you to make your choice.  The variations we both propose are different in nature. Tony Ennis suggests you go for a material advantage in exchange for the bishop pair (for what it is worth without convenient development). My variation also proposes a material advantage but doesn't surrender the bishop pair. Furthermore, my variation keeps the center solid (pawns in the center), avoiding counterplay and hindering free piece play on black's part. </p>

<p>The idea of d5 is thematic, of course, but premature and clearly inferior but it has one advantage: you are on your own and one single misstep and problems appear. It could be interesting in blitz play but not more, i think. The positions arising after d5 should be examined by you (and possibly others) with computer assistance to refute your missteps in analysis and to get an idea of how to handle various responses. </p>
 70: <p><code>3... d5</code> is not listed <a href="https://en.wikibooks.org/wiki/Chess_Opening_Theory/1._e4/1...e5/2._Nf3/2...Nc6/3._Bb5" rel="noreferrer">here</a> so it is a safe bet there's a problem.</p>

<p>It looks like an overreach, with both Black pawns being vulnerable.</p>

<p>I'd go for a pawn steal.</p>

<pre>
<code>
[FEN ""]

1. e4 e5 2. Nf3 Nc6 3. Bb5 d5
4.Nxe5 a6 5.Bxc6+ bxc6 6.Qe2 {White has better development, better pawns, and a free pawn.}
</code>
</pre>
