In an E2 elimination from an acyclic alkyl halide, the preferred spatial arrangement of the beta C-H bond and the C-X bond is:

The E2 transition state needs the C-H bond being broken and the C-X bond to lie in one plane so that the p orbitals of the new pi bond overlap well. The staggered anti-periplanar arrangement ($180^{\circ }$) is lowest in energy and is preferred. Syn elimination ($0^{\circ }$) is eclipsed and seen only in rigid systems.