Let $U\subsetneq\mathbb R^n$ be bounded and open. Suppose $a_{ij},b_i,c\in L^\infty(U)$, and let $B:H_0^1(U)\times H_0^1(U)\to\mathbb R$ be the [bilinear form](/page/Bilinear%20Form) associated to the divergence form operator \begin{align*} Lu&=-\sum_{i,j=1}^n\partial_{x_i}(a_{ij}\partial_{x_j}u)+\sum_{i=1}^n b_i\partial_{x_i}u+c u. \end{align*} Then there exists $C>0$ such that \begin{align*} |B[u,v]|&\le C\|u\|_{H^1(U)}\|v\|_{H^1(U)} \end{align*} for every $u,v\in H_0^1(U)$.