A fundamental process of surface energy minimization is the decay of a wire into separate droplets initiated by the Rayleigh-Plateau instability. Here we study the linear stability of a wire deposited on a unidirectionally patterned substrate with the wire being aligned with the pattern. We show that the wire is stable when a criterion that involves its width and the local geometry of the substrate at the triple line is fulfilled. We present this criterion for an arbitrary shape of the substrate and then give explicit examples. Our result is rationalized using a correspondence between the Rayleigh-Plateau instability and the spinodal decomposition. This work provides a theoretical tool for an appropriate design of the substrate's pattern in order to achieve stable wires of, in principle, arbitrary widths.