  We present a new description of nonequilibrium square patterns as a
harmonically coupled crystal lattice. In a vertically oscillating granular
layer, different transverse normal modes of the granular square-lattice pattern
are observed for different driving frequencies ($f_d$) and accelerations. The
amplitude of a mode can be further excited by either frequency modulation of
$f_d$ or reduction of friction between the grains and the plate. When the mode
amplitude becomes large, the lattice melts (disorders), in accord with the
Lindemann criterion for melting in two-dimensions.
