While all four matches thus far have been played to a draw, they have still been thrilling, with both players trading turns working themselves out of unfavourable positions to keep the match level.
But on Tuesday, the biggest development of the match might have taken place, and it occurred far away from any of the action on the chess board.
The Saint Louis Chess Clubs, essentially the home of American chess, posted, and soon-after, deleted a video on YouTube that appeared to show some of Caruana's preparation for his match against Carlsen.
This morning a 2-minute clip from one of Caruana's training camps was uploaded on YouTube (now deleted). It featured various activities, chess included. Viewers could also see a laptop screen with a ChessBase file laid open. The greatest intel blunder in chess history or a hoax? pic.twitter.com/nwHL75M2cC— Olimpiu G. Urcan (@olimpiuurcan) November 13, 2018
While the video might have looked harmless enough to the untrained eye, to many in the chess world the bits of information revealed could potentially be like giving an opposing team a peek at your playbook. Just glimpses of previous games Caruana and his team had been studying could give away ideas towards his plans for the rest of the match.
But though some saw the video as a huge mistake on the part of Caruana's team, others suggested it's possible that the challenger was taking part in a next-level mind game in order to trick Carlsen into a certain line of thinking.
When asked about the video after their fourth game of the match, Caruana declined to comment, while Carlsen earned laughs from the media by admitting, yes, he'd be looking at the video.
As things stand, we don't yet know if the clips of the video were an elaborate attempt at getting inside Carlsen's head or a costly mistake that gave away Caruana's game plan.
The result of Game 5 on Thursday will be telling.
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