We provide proofs and commentary on parts of Bruno De Finetti’s paper ”About Correlation”. In particular
we focus on De Finetti’s angle theorem, which he stated without proof and whose striking simplicity and
power would seem to make it a natural candidate for any multivariate statistics text. We establish a connection
between de Finetti’s angle theorem and ongoing research on the boundary of the convex set of correlation
matrices. We also provide evidence from this article that de Finetti discovered rank correlation before or at
the same time as Kendall.