We give sharp conformal conditions for the differentiability in the Sobolev space
W_{\mathrm {loc}}^{1,n-1}(\Omega, \mathbb R^{n})
. Furthermore, we show that the space
W_{\mathrm {loc}}^{1,n-1}(\Omega, \mathbb R^{n})
can be considered as the borderline space for some capacitary inequalities.