The permitted standing-wave modes can
be represented as a lattice in the
positive octant of the space whose axes are $n_{\rm x}$, $n_{\rm y}$,
and $n_{\rm z}$. Each point of the lattice represents one possible mode
of equilibrium cavity radiation. The space density of points in this
lattice is unity, so the number of points in any volume is equal to
that volume.