Using a consistent purely kinetic model we show that Young's condition at a trijunction results from a chemical and not a mechanical equilibrium, as is often believed in the literature. In an out-of equilibrium situation this condition is altered in a way such that the triple point acquires an effective mobility. The order of magnitude of the mobility is fixed by a velocity scale $\tilde{V}_{\rm att}$ associated with chemical attachment at the interface, and not by the sound speed. While for ordinary eutectics $\tilde{V}_{\rm att}$ is large (except in rapid solidification experiments) in comparison to the growth speed, implying a quasi-instantaneous mobility, finite mobility effects should show up for faceted eutectics as well as for eutectoid transformations.