Combinatorics (combos)
Combinatorics, or combo counting, is the practice of counting the exact card combinations a hand or range can have, turning a vague read into an actual number.
Combinatorics turns a hand or range into an exact number of card combinations instead of a feeling. Two baseline counts are worth memorising, because every estimate builds on them:
Pocket pair = 6 combos
Unpaired hand = 16 combos total (4 suited, 12 offsuit)
Every card that becomes visible, whether it is in your own hand or on the board, removes the combos that needed it. That is combinatorics doing its real work: turning a hunch into a count you can actually compare.
Worked example. Pocket aces exist in 6 combos before any cards are seen. If the flop contains one ace, only 3 aces remain unseen, so the opponent's possible pocket aces drop from 6 combos to 3. Hold one of the four aces yourself instead of seeing it on the board, and the same kind of drop happens elsewhere: the opponent's Ace-King holdings fall from 16 combos to 12, because only 3 aces are left for them to pair with a king.