Multiplicity of phase states within frequency locked bands in periodically
forced oscillatory systems may give rise to front structures separating states
with different phases. A new front instability is found within bands where
ωforcing/ωsystem=2n (n>1). Stationary fronts shifting the
oscillation phase by π lose stability below a critical forcing strength and
decompose into n traveling fronts each shifting the phase by π/n. The
instability designates a transition from stationary two-phase patterns to
traveling n-phase patterns