CO 100bb Defense

Overview

CO 100bb Defense refers to the optimal range and strategy for playing from the Big Blind (BB) against a Cutoff (CO) open-raise when both players have effective stacks of 100 big blinds (bb). This scenario is common in cash games and late tournament stages. The BB’s goal is to defend profitably by either calling or 3-betting, preventing the CO from auto-profiting with a wide opening range.

Key Principles

• Positional Disadvantage: The BB acts postflop out of position against the CO, which reduces the value of marginal hands. Therefore, the defense range is tighter than the CO’s opening range.
• Minimum Defense Frequency (MDF): To prevent the CO from profitably opening any two cards, the BB must defend a certain percentage of hands. With 100bb stacks and typical pot odds, the BB should defend around 50-60% of the time, including calls and 3-bets.
• Range Construction: A standard CO opening range is about 30-40% of hands (e.g., suited connectors, Broadway cards, pairs). The BB’s defense range includes:
  • Value 3-bets: Hands that dominate the CO’s calling range (e.g., TT+, AQ+).
  • Calls: Hands with good playability out of position (e.g., suited aces, suited connectors, medium pairs).
  • Bluff 3-bets: Hands that block the CO’s strong holdings and have decent equity (e.g., A5s, K9s, suited gappers).

Example Range (Approximate)

• Call: 22-99, A2s-A9s, ATo, KJs-KTs, QJs, JTs, T9s, 98s, 87s, 76s (about 15-20% of hands).
• 3-bet (value): TT+, AQ+, KQs (about 4-5%).
• 3-bet (bluff): A5s-A2s, K9s, Q9s, J9s, T8s, 97s, 86s, 75s (about 5-7%).
• Total defense: ~25-30% of hands (note: this is less than MDF suggests because of positional disadvantage; actual frequencies in practice often fall below theoretical MDF).

Adjustments

• Against a tight CO: Reduce calling range and increase 3-bet bluff frequency.
• Against a loose CO: Widen defense, especially with suited hands that flop well.
• Stack Depth: At 100bb, postflop play is important. Avoid calling with hands that are difficult to play out of position (e.g., offsuit weak aces).

Common Mistakes

• Over-folding: Allowing the CO to profitably steal blinds too often.
• Calling too wide: Playing weak offsuit hands out of position leads to losses postflop.
• Inflexible 3-betting: Using the same range regardless of opponent tendencies.

Conclusion

CO 100bb Defense requires a balanced approach mixing calls and 3-bets. The BB must defend enough to prevent exploitation but avoid playing too many hands that lose money out of position. Understanding stack depth, opponent, and postflop playability is critical.