  We define the notion of an aligned null direction, a Lorentz-signature
analogue of the eigenvector concept that is valid for arbitrary tensor types.
The set of aligned null directions is described by a a system of alignment
polynomials whose coefficients are derived from the components of the tensor.
The algebraic properties of the alignment polynomials can be used to classify
the corresponding tensors and to put them into normal form. The alignment
classification paradigm is illustrated with a discussion of bivectors and of
Weyl-type tensors. Note: an earlier version of this manuscript was published in
the proceedings of SPT 2004. The present version has been expanded to include a
discussion of complexified alignment. Section 4 also corrects errors contained
in the earlier manuscript.
