  {\alpha}-CaCr2O4 is a distorted triangular antiferromagnet. The magnetic Cr3+
ions which have spin-3/2 and interact with their nearest neighbors via
Heisenberg direct exchange interactions, develop long-range magnetic order
below T_N=42.6 K. Powder and single-crystal neutron diffraction reveal a
helical magnetic structure with ordering wavevector k=(0,~1/3,0) and angles
close to 120{\deg} between neighboring spins. Spherical neutron polarimetry
unambiguously proves that the spins lie in the ac plane perpendicular to k. The
magnetic structure is therefore that expected for an ideal triangular
antiferromagnet where all nearest neighbor interactions are equal, in spite of
the fact that {\alpha}-CaCr2O4 is distorted with two inequivalent Cr3+ ions and
four different nearest neighbor interactions. By simulating the magnetic order
as a function of these four interactions it is found that the special pattern
of interactions in {\alpha}-CaCr2O4 stabilizes 120{\deg} helical order for a
large range of exchange interactions.
