If X1:Ω→Ω′X_{1}: \Omega \to \Omega ' and X2:Ω′→Ω′′X_{2}: \Omega ' \to \Omega '' are measurable then the composition X2∘X1:Ω→Ω′′X_{2} \circ X_{1}: \Omega \to \Omega '' is also measurable.