  A version of the Davis-Kahan Tan $2\Theta$ theorem [SIAM J. Numer. Anal.
\textbf{7} (1970), 1 -- 46] for not necessarily semibounded linear operators
defined by quadratic forms is proven. This theorem generalizes a recent result
by Motovilov and Selin [Integr. Equat. Oper. Theory \textbf{56} (2006), 511 --
542].
