From the Terms of Service:
(b) While a game is in progress you may not refer to chess engines, chess computers or be assisted by a third party. Endgame tablebases may not be consulted during play but you may reference books, databases consisting of previously played games between human players, and other pre-existing research materials.
So the short answer is yes; and I use both chessgames.com and chesslive.de
But there's a catch: chessgames.com contains a very small percentage of games played by computers. The percentage is so small that only once have I stumbled across one. I continue to use this database, however, since I believe the percentage to be so small that the database as a whole ought to be acceptable.
I would welcome an authoratitive ruling on this.
Failing that, if chessgames.com is not acceptable then can anyone recommend an openings database that is guaranteed to be 100% human?
So we can use the opening databases during play then? The excerpt from the rules presented during login did not include the part of the phrase about referencing pre-existing material, I don't think, or I didn't read it properly. But it is still in the terms.
I frequently consult the opening books because it helps me not to stuff up the games until much later.
😕
Unfortunately the index behind the games explorer is still a case-insensitive index by FEN. That is, it doesn't know the difference between a white pawn and a black pawn.
This is explained on page 7 of the Games Explorer thread Thread 57414. As a quick demonstration try the "cxd4" link here:
http://www.redhotpawn.com/gamesexplorer/index.php?movelist=e2e4e7e6d2d4d7d5b1c3c7c5e4d5&flip=0&co=-&u=-1&c=1400
This kind of thing is surely going to make the percentage statistics unreliable as soon as any pawns reach the fifth rank. The same bug affects www.gensunasumus.com and caused me a lot of grief when I was using that site in 2005/2006.
But once that's been fixed it will be a strong contender.