If for all t∈Tt \in T we have Xt∈ℬ/ℬ′X_t \in \mathcal{B}/\mathcal{B}' then we define σ(Xt,t∈T)=⋁t∈Tσ(Xt)\sigma(X_t,\, t \in T) = \bigvee_{t \in T} \sigma(X_t).