A two-event probabilistic situation with events and is a single mathematical object — the four joint probabilities that sum to — that we can observe from three perspectives: the Venn diagram (visual and symmetric), the contingency table (compact) and the tree diagram (sequential and conditional). The three representations encode the same information and convert into one another through six elementary steps (plus a seventh conversion, Bayes’ inverted tree). Mastering these steps is the operational key to almost any exercise.