Which opening is the longest?

Of all the opening postions available here at chess.com, which one starts you off the furthest into the game?  The longest one I've found so far is 18 moves (the game starts on white's 19th move):

C89 Ruy Lopez: Marshall, Main line, Spassky Variation
1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.O-O Be7 6.Re1 b5 7.Bb3 O-O 8.c3 d5 9.exd5 Nxd5 10.Nxe5 Nxe5 11.Rxe5 c6 12.d4 Bd6 13.Re1 Qh4 14.g3 Qh3 15.Be3 Bg4 16.Qd3 Rae8 17.Nd2 Re6 18.a4 Qh5

Is there one that's longer?

Its misleading to say that at move 18 or 19 its still in the opening, as its clearly in the middlegame.

But with this line, you can actually BEGIN the game at move 19 (using Chess.com's predetermined opening positions).  I was just wondering if this is the longest "book" line.

It might be the longest of chess.coms thematic starting positions, not sure. However there are lines in " opening books " that go beyond move 30.

     If you click the top moves of the Sicilian on Game Explorer I believe it
goes past 18 moves.

I would bet that there's a Sicilian that has the honor of being the longest thematic (as Reb says, once you get into the middle game, it really should no longer be called an opening, because it is reachable from more than one opening).

The Main lines of the Zaitsev Variation of the Ruy Lopez start around move 18, and go atleast another 18 moves in some lines. The semi-slav Botvinnik system is analyzed down to an endgame in many variations. I even read that one line in the 9.Bc4 yugoslav attack is analyzed so far that the king and pawn endgame resulting from it has been solved. Opening theory can basically go as far as you want it to go. With the ever increasing strength of computer programs, combined with deep analysis done by world-class players, many chess openings may one day be "solved".  Perhaps instead of calling an opening the "Sicilian Najdorf", once both sides have worked out the very best possible moves, the whole game after 5.a6 will be called the "Sicilian Najdorf" and only have one line, that is completely solved. Or even worse, the whole "Sicilian" will become one line that runs 94 moves deep, and has been worked out to a drawn king and pawn endgame. I hope not, because if the very best moves from move 1 were worked out for both sides and the game was a draw, than top level chess would become very boring, or non-existent.

Fortunately, most of us don't need to know every line 30+ moves deep. Make sure you know the sharp critical lines of the openings you play, but most importantly, know the plans and structures of the middlegames and endgames that they lead to.

There is a thin line between the openning and the middle game. I do not think that you can give an exact move as the one that ends the opening and starts the middle game.

Tell us what it is?

it is why somebody like chess960, there is no openning issue - no need to remember so much variations. 

Ok, so with all that said, does anyone know of an opening position available on here that starts you off further into the game than this one (that was the original question )?

C89 Ruy Lopez: Marshall, Main line, Spassky Variation
1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.O-O Be7 6.Re1 b5 7.Bb3 O-O 8.c3 d5 9.exd5 Nxd5 10.Nxe5 Nxe5 11.Rxe5 c6 12.d4 Bd6 13.Re1 Qh4 14.g3 Qh3 15.Be3 Bg4 16.Qd3 Rae8 17.Nd2 Re6 18.a4 Qh5

There's 150 different positions just in ECO A00 alone.  No matter how you do the search, it will only show you the first 2000 positions, so I don't even know how many there are!!!  Sorted by ECO, 2000 gets you up to C41.  Sorted Alphabetically, at 2000 you're just starting to get into the Queen's Pawn games.  And sorted by move, by 2000, you're still at 1.e4, e5.  So I'm guessing there's probably between 4000-5000 different opening positions available here.  WOW!!!    No wonder I haven't been able to look thru all of them yet. 

Yeah, on here, if you keep clicking the most popular move, you can get to move 22.  On other web sites you can get somewhere between 13-25.  I'm on the side that calls these "book" moves, and once the database runs out I say we're "out of book" even tho some argue that "book" lines are those that have theory written about them.  Either way, if you click thru the top moves, the last place you'll see an opening position is after black's 9th move, so the game would start at white's 10th move.

I don't fully understand.  Why is white's 10th move not an opening position?

When you start a game here at Chess.com (it doesn't matter if it's a personal game, a Team Match, or Vote Chess), you have the option of starting from the standard position, or a set of predetermined openings all named and listed by their ECO code (Encyclopedia of Chess Openings).  In the line we mentioned, you can START the game at move 10 by selecting B99: Sicilian Defense: Najdorf Variation, Main Line as the opening position. 

Just because I can follow all of Bobby Fishers games from move one to the end, doesn't mean that every position is listed in the ECO.  Databases will often take you much further, which is another reason some don't like to refer to them as "book" or "opening" moves.

     I see what you mean now.  I have actually memorized that line out to 15
moves, but the move 10. g4 is not part of the actual ECO.
     So back to your original question, that the 18 moves are all part of the ECO.  I
get it now.  Thanks for the explanation.
     By the way, "The Who" still rocked the Super Bowl.  They only had exactly
12 minutes to work with and of course they're still not in their prime, but I would still rather watch them than most modern bands!!!

LOL.  You're right of course.  The Who at 75-80% is still better than a lot of bands at 100%.

to find the answer to what opening is the longest you must first find the answer to where does openings stop and middlegames begin since there is no clear definition to that. good luck with that one:)

Actually, to my question there is a very clear definition.  If it is listed in Chess.com's database of opening positions, it counts.  If it's not, it doesn't.  Which Opening Position starts the furthest into the game?

Right! If I understand correctly, the question is asking; which of the chess.com presets will start a game furthest into the game; whether you concider it the opening or middlegame or endgame is irrelevent.

They only call it opening theory that deep because it stems from openings and they need to know that well if they're playing that opening.