A tuned-mass damper (TMD) is a specially-designed structural/mechanical element that can be incorporated into a new structure during the design phase or added to an existing structure to reduce the vibration of the structure. TMDs are most effective where the structure's motion is caused by one or more resonant modes of the original structure. In essence, the TMD extracts vibration energy (i.e., adds damping) to the structural mode it is "tuned" to. The end result: the structure feels much more stiff than it actually is.