They are produced due to the interference of two identical progressive waves traveling along the same path but in opposite directions. They move neither forward nor backward. In a stationary wave, the energy is not transported from one point to another The particles of the medium perform S. H. M. of the same period but the amplitude of the oscillations varies periodically in space. There are some particles of the medium which do not at all vibrate i.e. their amplitude is zero. These are called nodes. Some particles vibrate with maximum amplitude and are called antinodes.