How to read this diagram: The total square represents all possibilities. The width of the left column is c(H) — the prior probability of the hypothesis. Within each column, the height of the top region is the probability of a positive report (says H) given that column's truth value. The highlighted regions are where the evidence says H. The posterior c(H | says H) is the ratio of the dark blue area (H true and says H) to the total highlighted area (everything that says H).