shows a simple pendulum. In most practical high-rise structures, such a system also has a simple problem: too much length. To avoid that, MHI uses a multistage pendulum. It effectively folds the length by interposing shorter rope lengths that can pivot and lengths of frame that cannot. As Figure 1 shows, a 3-stage system reduces the needed length to one third that of a simple pendulum, thus keeping system headspace needs low. Now, if the mass can be driven in the pendulum system in such a way that its motion counteracts the forces of building sway, the amplitude of structural sway will be reduced.