A Kelvin wave is a kind of wave in a rotating system that relies on gravity for its restitutive force, and is called a boundary wave because it transmits along the boundary of the fluid. The amplitude of the wave decreases exponentially with a distance from the boundary (or wall). In the case of inertial gravity waves, which are another kind of gravity wave, Coriolis' force forms part of the restitutive force, whereas Kelvin waves use Coriolis' force to push against the wall, and not for restitutive force. The movement of the particles of the fluid in a Kelvin wave is always parallel to the wall, so the Coriolis' force which acts on them balances out with the pressure gradient perpendicular to the wall. In other words, Coriolis' force does not act parallel to the wall, so the period of a Kelvin wave becomes the same as that of a stationary system.