The self-thinning rule describes plant mortality because of competition in crowded even-aged stands. The rule is best understood with respect to a graph of log biomass (log B) per unit area vs. log density (log N) of survivors, known as the B–N diagram. The rule has three notable features—(1) mortality is a function only of biomass accumulation, (2) because mortality is driven by the rate of accumulation of biomass, mortality is slower when conditions for growth are worse, and (3) the thinning line has a slope of about –½ for most studied species under most conditions. Two main effects operate in developing distributions in even-aged stands. First, large plants suppress small plants. The result is a “hierarchy of dominance and suppression” in which the smaller plants are at an accumulating disadvantage and finally die. Second, the mortality of smaller individuals truncates the distribution from the left.