The triangle coefficient is function of three variables written Δ(a b c) congruent Δ(a, b, c) and defined by Δ(a b c) congruent ((a + b - c)!(a - b + c)!(-a + b + c)!)/((a + b + c + 1)!), (Shore and Menzel 1968, p. 273), where z! is a factorial. It arises for example in the definition of the Wigner 6j-symbol.