The purpose of this note is to bring into attention an apparently forgotten
result of C. M. Petty: a convex body has minimal surface area among its af
fine transformations of the same volume if, and only if, its area measure i
s isotropic. We obtain sharp affine inequalities which demonstrate the fact
that this "surface isotropic" position is a natural framework for the stud
y of hyperplane projections of convex bodies.