When constructing theories such as QED which is invariant under space parity and time reversal, Dirac spinors may be used, while theories that do not, such as the electroweak force, must be formulated in terms of Weyl spinors. The Dirac representation, (⁠ 1 / 2 ⁠, 0) ⊕ (0, ⁠ 1 / 2 ⁠), is usually taken to include both space parity and time inversions. Without space parity inversion, it is not an irreducible representation.