  The paper addresses the doubly elliptic eigenvalue problem $$\begin{cases}
-\Delta u=\lambda u \qquad &\text{in $\Omega$,}\\ u=0 &\text{on $\Gamma_0$,}\\
-\Delta_\Gamma u +\partial_\nu u =\lambda u\qquad &\text{on $\Gamma_1$,}
\end{cases} $$ where $\Omega$ is a bounded open subset of $\mathbb{R}^N$ ($N\ge
2$) with $C^1$ boundary $\Gamma=\Gamma_0\cup\Gamma_1$,
$\Gamma_0\cap\Gamma_1=\emptyset$, $\Gamma_1$ being nonempty and relatively open
on $\Gamma$. Moreover
$\mathcal{H}^{N-1}(\overline{\Gamma}_0\cap\overline{\Gamma}_1)=0$ and
$\mathcal{H}^{N-1}(\Gamma_0)>0$. We recognize that $L^2(\Omega)\times
L^2(\Gamma_1)$ admits a Hilbert basis of eigenfunctions of the problem and we
describe the eigenvalues. Moreover, when $\Gamma$ is at least $C^2$ and
$\overline{\Gamma}_0\cap\overline{\Gamma}_1=\emptyset$, we give several
qualitative properties of the eigenfunctions.
