text: In a one-dimensional system at a given mode the vibration will have nodes, or places where the displacement is always zero.  These nodes correspond to points in the mode shape where the mode shape is zero.  Since the vibration of a system is given by the mode shape multiplied by a time function, the displacement of the node points remain zero at all times.
  When expanded to a two dimensional system, these nodes become lines where the displacement is always zero.  If you watch the animation above you will see two circles (one about halfway between the edge and center, and the other on the edge itself) and a straight line bisecting the disk, where the displacement is close to zero.  In an idealized system these lines equal zero exactly, as shown to the right.