  In this paper, we establish suitable characterisations for a pair of
functions $(W(x),H(x))$ on a bounded, connected domain $\Omega \subset
\mathbb{R}^n$ in order to have the following Hardy inequality
  \begin{equation*}
  \int_{\Omega} W(x) |\nabla u|_A^2 dx \geq \int_{\Omega} |\nabla
d|^2_AH(x)|u|^2 dx, \,\,\, u \in C^{1}_0(\Omega),
  \end{equation*}
  where $d(x)$ is a suitable quasi-norm (gauge), $|\xi|^2_A = \langle A(x)\xi,
\xi \rangle$ for $\xi \in \mathbb{R}^n$ and $A(x)$ is an $n\times n$ symmetric,
uniformly positive definite matrix defined on a bounded domain $\Omega \subset
\mathbb{R}^n$. We also give its $L^p$ analogue. As a consequence, we present
examples for a standard Laplacian on $\mathbb{R}^n$, Baouendi-Grushin operator,
and sub-Laplacians on the Heisenberg group, the Engel group and the Cartan
group. Those kind of characterisations for a pair of functions $(W(x),H(x))$
are obtained also for the Rellich inequality. These results answer the open
problems of Ghoussoub-Moradifam \cite{GM_book}.
