There's something I don't understand about the Openings Explorer

Very often this happens: I reach a certain position in the openings explorer, and it lists some continuations with, for example, just one game recorded that demonstrates that continuation. But then, when I put that continuation in as the next move, suddenly there are mutliple recorded games continuing from there! Why didn't they show up before? Am I misunderstanding something?

It all has to do with transpositions.

Let's say, hypothetically, that only 5 chess games were ever played, and they started as follows:

1.e4 e6 2.d4 d5 3.Nc3 Nf6 4.Bg5

1.e4 e6 2.d4 d5 3.e5 c5 4.c3

1.d4 e6 2.e4 d5 3.Nc3 Nf6 4.Bg5

1.d4 e6 2.c4 f5

1.d4 Nf6 2.Nc3 d5 3.Bg5 e6 4.e4

If you move 1.e4, it will say 1...e6 has occurred twice, games 1 and 2 in the database.

You move 1...e6. Now it will say 2.d4 has occurred 3 times because the position after 2.d4 occurred in games 1, 2, AND 3. Then after 2.d4, it will show 2...d5 with a 3.

Now it will show 3.Nc3 with a 2 and 3.e5 with a 1. You play 3.Nc3.

Now it will show 3...Nf6 with a 2 because the position after 3...Nf6 occurred twice.

Lastly 4.Bg5 will now show a 3 because games 1, 3, AND 5 feature the position after 4.Bg5 even though the last started as a Veresov.

Hope this explains it. Now picture it with millions of games instead of 5.

And so if you played, say, the Veresov, and did 1.d4 Nf6 2.Nc3 d5 3.Bg5 e6, each of those moves on move 2 and 3 might be in the hundreds, but then 4.e4 and you are in the tens of thousands because of all the occurrences of the position from the French Defense. You technically are no longer in a Veresov.

Great explanation, thanks. On the one hand I wonder why they don't just give the number of games where the resulting position occurred rather than the number of games in which that move was made. On the other hand it's not that big a deal to click through and find out for myself.

To answer post 4, since he embedded it in the quote rather than in his own message, the reason is simple.

The point of the count is not to indicate how many times a move is played from a specific position. It is to indicate how many times the position that would result from that move has occurred, no matter how it got there.

So after 1.e4 e6 2.d4 d5 3.Nc3 Nf6, whatever number shows after 4.Bg5 is how many times that POSITION has occurred, NOT how many times that move was played from this position.

The point is that it shows the total percentage for White, Black, and draw from that position as a whole.

Why should I care HOW it got there? The position is the same whether via the French or Veresov (1.d4 Nf6 2.Nc3 d5 3.Bg5 e6 4.e4)?

If that position occurred 5000 times, I want to know how White (white win percentage + 0.5*Draw percentage) and Black (Black win percentage + 0.5*Draw percentage) have scored across all 5000 games, not just the 4639 of them that came from this specific order of moves!