The physics of nutation in tops and gyroscopes can be explored using the model of a "heavy symmetrical top" with its tip fixed. (A symmetrical top is one with rotational symmetry, or more generally one in which two of the three principal moments of inertia are equal.) Initially, the effect of friction is ignored. The motion of the top can be described by three Euler angles: the tilt angle between the symmetry axis of the top and the vertical; the azimuth of the top about the vertical; and the rotation angle of the top about its own axis. Thus, precession is the change in and nutation is the change in .