  We classify real trivectors in dimension 9. The corresponding classification
over the field C of complex numbers was obtained by Vinberg and Elashvili in
1978. One of the main tools used for their classification was the construction
of the representation of SL(9,C) on the space of complex trivectors of C^9 as a
theta-representation corresponding to a Z/3Z-grading of the simple complex Lie
algebra of type E_8. This divides the trivectors into three groups: nilpotent,
semisimple, and mixed trivectors. Our classification follows the same pattern.
We use Galois cohomology, first and second, to obtain the classification over
R.
