  A condition on an affine central subalgebra $Z$ of a noetherian algebra $A$
of finite Gelfand-Kirillov dimension, which we call here \emph{unruffledness},
is shown to be equivalent in some circumstances to the flatness of $A$ as a
$Z$-module. Unruffledness was studied by Borho and Joseph in work on enveloping
algebras of complex semisimple Lie algebras, and we discuss applications of our
result to enveloping algebras, as well as beginning the study of this condition
for more general algebras.
