  We study linear perturbations about a dust filled Bianchi Type I model with
the vorticity set to zero. In comparison to linear perturbations about FLRW
models, modes of perturbations about Bianchi type I models are coupled. We find
that the tensor that represents the background shear needs to be degenerate in
order for the scalar-mode perturbations to decouple from the rest of the flow.
