The inertia matrix or inertia tensor is $\left[I_{xx} -I_{xy} -I_{xz}\-I_{yx} I_{yy} -I_{yz} -I_{zx} -I_{zy} I_{zz}\right]$ In linear algebra it was shown that for some angle equation 55 can be transform into $other matrix$ System which creates equation 56 referred as principle system.