The Toyota Hybrid System THS / Hybrid Synergy Drive has a single power-split device (incorporated as a single three-shaft planetary gearset) and can be classified as an Input-Split, since the power of the engine is split at the input to the transmission. This in turn makes this setup very simple in mechanical terms, but has drawbacks of its own. For example, in Generation 1 and Generation 2 HSDs maximum speed is mainly limited by the speed of the smaller electric motor (often functioning as a generator). The Generation 3 HSD separates the ICE-MG1 path from the MG2 path, each with its own, tailored gear ratio (1.1:1 and 2.5:1, respectively, for late Priuses, including the Prius c). The Generation 4 HSD eliminates the second planetary gear set, and places the electric motors on parallel axes, with a combining gear in between these axes, and transfers the combined result to the final drive differential. This is quite similar to Toyota-affiliated Aisin Seiki's hybrid system, and saves significant space.