A game of Texas Hold'em has four betting rounds called the preflop, flop, turn and river. In the preflop every player receives two cards face-down (known only to the player) called hole or pocket cards, and a betting round ensues. During the flop three community or board cards are dealt face-up (known by all the players) and the second betting round follows. At the turn a fourth face-up community card is dealt, followed by a betting round. Finally, at the river, a fifth face-up community card is dealt and the last betting round occurs, followed by a showdown. In the showdown, the pot is awarded to the best five-card hand that an active player can make combining the hole cards and the five community cards. There is normally a maximum of three raises allowed per betting round. In Texas Hold'em, the number of players can vary from 2 to 23; but it is usually played with 8 to 10 players. Figure 2.1 shows a hand of Texas Hold'em (on the turn) from Loki's point of view against two opponents. The cards denoted by a single question mark represent the imperfect information of the game. The card denoted by two question marks represents the non-deterministic outcome in the game (the card to come).
The players are in a fixed seating order at the table. The dealer-button rotates clockwise around the table to indicate the (theoretical) dealer of each hand. The player to the immediate left of the button (the small blind) is first to receive a card. Figure 2.2 shows a table with the button's and blinds' positions indicated. Betting on the preflop starts with the player on the left of the big blind. On subsequent rounds, the first active player left of the button acts first (the small blind if not folded).
The betting structure of 2-4 Limit Texas Hold'em starts with two forced bets (blinds) on the preflop: a small blind of one unit and a big blind of two units. Blinds are a ``forced-bet'' alternative to the more familiar ante in other games, where each player is required to put a fixed amount into the pot before the game begins. In a 2-4 Texas Hold'em game, all bets and raises are a fixed size of two units during the preflop and flop. This doubles to four units for all bets and raises on the turn and river. Other amounts can also be used. For example, in a $10-$20 game, the unit size is $5.00. The small blind posts a $5 blind, the big one blinds $10. The size of the bet during the first two rounds (preflop and flop) is $10.00 and during the last two rounds (turn and river) is $20.00.
