  In this paper we study under what boundary conditions the inequality
$$\|\nabla\omega\|_{L^2(\Omega)}^2\leq C\left(\|{\rm
curl}\omega\|_{L^2(\Omega)}^2+ \|{\rm
div}\omega\|_{L^2(\Omega)}^2+\|\omega\|_{L^2(\Omega)}^2\right) $$ holds true.
It is known that such an estimate holds if either the tangential or normal
component of $\omega$ vanishes on the boundary $\partial\omega.$ We show that
the vanishing tangential component condition is a special case of a more
general one. In two dimensions we give an interpolation result between these
two classical boundary conditions.
