Pendulums have played an important role in the history of mechanical physics,
both in the theoretical and practical definition of equal periods (chronometrics)
and as examples of the applied principles of dynamics.

Physically, the coupled pendulums are characterised by a continuous exchange
of energy from one to the other, as shown in the motion diagrams provided.

Three cases are particularly interesting to observe:

1. One of the two pendulums is initially at rest. In this case, the swing of the
   second, moving pendulum is absorbed by the first until it is totally stopped,
   at which point the first reaches its maximum. Then the situation is reversed,
   and so on…
   
2. The two pendulums have the same movement (they are in phase).
   
3. The two pendulums are initially in opposite positions (they are in opposed phase)
   

Any other movement can be described as a combination of these two last cases.