What is Crossbook in Poker?

Crossbook or Crossbooking refers to a bet between players in the same tournament or cash game, in which the player who loses owes the winner a percentage of that player’s net winnings (vs. the losing player’s net winnings) in the event.

For example, If Doug and Ryan agree to crossbook for 10 percent of the WSOP Main Event, Doug cashes for $50,000 and Ryan cashes for $100,000, Doug owes Ryan $5,000 on the crossbook bet ($100,000 – $50,000 * 0.10).