“…Then nor X is H' l~l -measurable and locally (d -l)-integrable and, at H d~lalmost all (x. n) e nor X, the (generalized) principal directions a,(x,ri) e S d~l and (generalized) principal curvatures K,(X, ri) e (-00, 00] (/ = 1,..., d -1) are denned so that the (H''~\d -l)-approximate tangent cone Tan''" 1 (nor X, (x, ri)) is a {d -1 )-dimensional linear subspace of K 2 '' spanned by the vectors Remark. The analogous result for sets with positive reach was stated and used in [10]. We present a proof here for the sake of completeness.…”