The taller a building, the longer its natural period tends to be. Taller buildings tend to be more flexible than short buildings. If it is very short, it is difficulty to bend it in your hand. If the rod is somewhat longer, and of the same diameter, it becomes much easier to bend. We say that a short building is a stiff, while a taller building is flexible.
Stiffness greatly affects the building's uptake of earthquake generated force.
Reconsider our first example above, of the rigid stone block deeply founded in the soil. Real buildings, of course, are more inherently flexible, being composed of many different parts. To take once again the example of the rigid block in Figure 3, the block is an example of a structure with extremely low ductility. To see how ductility can improve a building's performance during an earthquake, consider Figure 4. For the block, we have substituted a combination of a metal rod and a weight.