A range is the full set of hands a player can hold in a given spot, given everything he has done so far. Thinking in ranges, instead of guessing one hand, is the basis of every postflop decision in this trainer.
Each action narrows a range. When the button opens and the big blind calls, the button’s range holds every big pair and most strong aces, while the big blind’s is wider and weaker, because he would have 3-bet many of his best hands. A flop call caps a range further: it mostly holds one pair, draws and floats, since sets and two pairs often raise. A check from the preflop raiser says even more — strong hands almost always bet, so his checking range is mostly medium pairs, ace-high and missed draws.
You also play your own range. A bet is made for the range, not just for the two cards you hold: with Q♣J♣ on A♥7♦2♣ you have nothing, yet the range bet includes it. The size you choose has to be the one your whole range would use here, or it tells villain exactly what you hold. And defense is a range rule too — MDF tells you how much of your range must continue, while your hand’s own equity decides whether it is part of that share.
Worked example
FAQ
How do you put someone on a range?
Start from his preflop action, then remove hands with every street: strong hands that would have raised are gone after a flop call, and strong hands that would have bet are gone after a check. What is left is the range you play against.
Why not just guess villain’s exact hand?
Because he can hold many hands, and your decision has to make money against all of them at once. A call that beats his most likely hand can still lose against the whole range if the price is wrong.