Current Jeopardy! champion James Holzhauer has proven that dominating the game is about more than what you know—it's also about what you wager. The professional gambler is known for making large, oddly specific bets on the show, a strategy that has earned him $1,275,587 in just 17 days. But there are five dollar amounts you won't see him wagering on Jeopardy!, as Mashable reports.

Ken Jennings, who took 74 days to win $3,270,700 (about 2.5 times Holzhauer's current earnings), recently joked about his sensible betting style on Twitter. The tweet led to a revelation about what contestants can and cannot wager on the show. According to Jennings (who also writes Mental Floss's Kennections column), betting exactly $69 "is officially forbidden on Jeopardy now, as of last year."

Holzhauer added in his in own tweet that, "Actually, there are five banned wagers," and the number 69 "is probably the most innocuous one." In addition to barring players from using Final Jeopardy as an opportunity to make a crude joke, the show also forbids them from betting $666, $14, $88, or $1488, former contestants confirmed on Reddit. The number 666 has satanic associations in Christianity, and 1488 is a white supremacist symbol.

Adhering to the wager rule is the least of Jeopardy! contestants' worries when they step up to the podium. Here are some secrets of the show's past winners.

[h/t Mashable]