The traditional convention of magnetic resonance envisions the paramagnets aligning with the external magnetic field; however, in practice it is simpler to treat the paramagnets as fixed and the external magnetic field as a vector. Specifying positional relationships requires three separate but related pieces of information: an origin, the distance from said origin, and a direction of that distance. The origin, for purposes of this explanation, can be thought of as the position of a molecule's localized unpaired electron. To determine the direction to the spin active nucleus from the localized unpaired electron (remember: unpaired electrons are, themselves, spin active) one employs the principle of magnetic angle selection. The exact value of θ is calculated as follows to the right: