Dutch mathematician and physicist Christiaan Huygens observed 1673 that two pendulums mounted on the same beam will come to swing in perfectly opposite directions, an observation he referred to as odd sympathy which in modern times is known as resonance. The tendency of two pendulums to synchronize, or asynchronize, is also refered to as the lock-in-effect. A beautiful example of this phenomenon is brought into being with metronomes in the following video.