Recently, Laub et al. [1] and Fernando and Nicholson [2] extended the definition of the cross-Gramian matrix

to include symmetric MIMO systems where the matrix

is defined as an integral. In this correspondence, we define this matrix as a solution of a Lyapunov equation and prove the fundamental identity

where

and

are the controllability and observability Gramians defined via Lyapunov equations. This definition of

complements the integral definition and has some advantages from theoretical and numerical points of view.