A de Finetti diagram is a ternary plot used in population genetics. It is named after the Italian statistician Bruno de Finetti (1906–1985) and is used to graph the genotype frequencies of populations, where there are two alleles and the population is diploid. It is based on an equilateral triangle, and Viviani's theorem concerning any point within the triangle, and the three lines from that point that are perpendicular to the sides of the triangle. The sum of the lengths of the lines is a fixed value, regardless of the position of the point. This value (the sum of the lengths) is set to be 1.