  We present the complete classification of equations of the form
$u_{xy}=f(u,u_x,u_y)$ and the Klein-Gordon equations $v_{xy}=F(v)$ connected
with one another by differential substitutions $v=\varphi(u,u_x,u_y)$ such that
$\varphi_{u_x}\varphi_{u_y}\neq 0$ over the ring of complex-valued variables.
