The irreducible representations are labeled by a pair of half-integers or integers (A, B). From these all other representations can be built up using a variety of standard methods, like taking tensor products and direct sums. In particular, space-time itself constitutes a 4-vector representation (⁠ 1 / 2 ⁠, ⁠ 1 / 2 ⁠) so that Λ ∈ D. To put this into context; Dirac spinors transform under the (⁠ 1 / 2 ⁠, 0) ⊕ (0, ⁠ 1 / 2 ⁠) representation. In general, the (A, B) representation space has subspaces that under the subgroup of spatial rotations, SO(3), transform irreducibly like objects of spin j, where each allowed value: j = A + B , A + B − − 1 , … … , | A − − B | , occurs exactly once. In general, tensor products of irreducible representations are reducible; they decompose as direct sums of irreducible representations.