This chapter will be more relaxing: SB defense ranges versus a BTN open are among the simplest, both in GTO (this chapter) and in Exploitative play (next chapter).
The Small Blind defends very little — being out of position, facing a strong BTN range, and under the threat of a BB squeeze — so its strategy is almost always reduced to 3-bet shove or Fold.
The only subtleties appear between 20 and 25 BB, where we find a few 3-bet sizes (Non All-in 3-bets). But once we simplify the combos, it becomes very easy to remember:
Note that in our GTO ranges, we have intentionally removed the calling range.
Don’t worry: this won’t impact your EV, but it will make the ranges much easier to memorize and apply.
🛡️ Also note that if you’re not comfortable using a 3-bet size versus a regular player — especially as a bluff — you can simply fold without stress; it will barely affect your EV.
In this very unfavorable position, it is generally safer to “slightly under-defend” than to over-defend.
There isn’t much more of importance to say about this position. As usual, if you want to go a bit deeper, I recommend checking the Strategy Guide for SB vs BTN open.