http://en.wikipedia.org/wiki/Fixed_limit
Straddle bets
A straddle bet is an optional (voluntary) blind bet made by a player before receiving his cards. Straddles are only used in games played with blind structures. Straddles are normally not permitted in tournament formats.
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Live straddle
The player immediately to the left of the big blind may place a live straddle blind bet. The straddle must be a raise over the big blind. A straddle is a live bet; the player placing the straddle effectively becomes the "bigger blind". Action begins with the player to the left of the straddle. If action returns to the straddle without a raise, the straddle has the option to raise. The player to the left of a live straddle may re-straddle by placing a blind bet raising the original straddle. [4][3]
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Mississippi straddle
A Mississippi straddle buys last action before the flop. House rules permitting Mississippi straddles are common in the southern United States. Usually, a Mississippi straddle can be made from any position, although some house rules only permit the button or the player to the right of the button to place a Mississippi straddle. Like a live straddle, a Mississippi straddle must be at least the minimum raise. Action begins with the player to the left of the straddle. If, for example (in a game with $10-$25 blinds), the button puts a live $50 on it, the first player to act would be the small blind, followed by the big blind, and so on. If action gets back to the straddle with no raise, the straddle has the option of raising. The player to the right of a Mississippi straddle may re-straddle by placing a blind bet raising the original straddle. [5]
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Sleepers
A sleeper is a blind raise placed from any position at the table other than under the gun.[6]