In experiments we are concerned with finding the g-tensor, as can be seen from the Hamiltonian. The PAS of the first step consists out of irradiating the solid sample with electrons so defects will be formed with unpaired electrons. These will produce our EPR signals. To determine PAS of the g-tensor, we need to rotate the sample in different directions so the radiation will go through several planes of the solid, these are indicated with ,  etc. This way, we can find an orientation where the g-tensor is diagonal and expressible via its zz-component.