CO 10bb Defense

Overview

In tournament poker, the term CO 10bb Defense refers to the decision-making process for a player in the cutoff (CO) position with an effective stack of 10 big blinds (bb) when an opponent in an earlier position (e.g., UTG, MP) opens the pot. At this stack depth, the standard preflop strategy shifts from raise-fold to push-fold, meaning that calling or making small raises becomes suboptimal due to the risk of being forced to fold to a re-raise and losing significant equity. Instead, the CO typically responds by either folding or moving all-in, with calling considered only in rare ICM-specific situations.

Stack Size Dynamics

With 10bb, the CO's post-flop playability is extremely limited. A standard open-raise of 2-2.5bb would commit over 20% of the stack, leaving the CO with a weak SPR (stack-to-pot ratio) and vulnerable to c-bets. Therefore, the CO's defense must be all-in or fold. The precise range depends on factors such as the opponent's position, the table's ICM pressure (e.g., near the bubble or final table), and the opponent's tendencies.

Example Ranges

While ranges vary by solver output, a typical GTO-oriented CO 10bb defense against an UTG open might include:

• Push all-in: Hands like TT+, AJ+, KQ, and sometimes suited connectors like 76s as bluffs.
• Call: Rarely used, but could include hands like 99 or AQ when the opponent is known to fold to 3-bets or when ICM dictates preserving stack.
• Fold: Most weak aces (A2-A9), small pairs (22-66), and unsuited broadways.

Against a MP open, the range widens slightly due to less perceived strength, including more pocket pairs and suited aces.

ICM Considerations

ICM can heavily alter the defense range. For example, on the bubble with equal stacks, the CO's push range narrows to only premium hands because losing doubles up a short stack near the money. Conversely, with a skill edge, the CO might expand the range to apply pressure.

Alternative Interpretation: Defending the Big Blind

In some contexts, "CO 10bb Defense" could describe the big blind's defending range when the cutoff opens with 10bb. However, this is less common; the standard term is "BB vs CO 10bb Defense." The core principle remains: the big blind also shifts to push-fold when facing a raise from a short-stacked cutoff.

Strategic Takeaways

• Never call: With 10bb in the cutoff, calling an open-raise is almost always a mistake unless there is a very specific ICM payout that makes calling then folding to a jam better than jamming yourself.
• Adjust to opponent: If the raiser is tight, expand the push range; if loose, tighten it.
• Know the bubble: ICM pressure should tighten ranges, while early in a tournament you can be more aggressive.

Understanding CO 10bb defense is critical for tournament players to avoid losing equity through passive play and to exploit opponents who play incorrectly.