The mathematics of isospin was modeled after that of spin. Isospin projections varied in increments of 1 just like those of spin, and to each projection was associated a "charged state". Since the "Delta particle" had four "charged states", it was said to be of isospin "I" = . Its "charged states" , , , and , corresponded to the isospin projections "I"3 = +, "I"3 = +, "I"3 = −, and "I"3 = −, respectively. Another example is the "nucleon particle". As there were two nucleon "charged states", it was said to be of isospin . The positive nucleon (proton) was identified with "I"3 = + and the neutral nucleon (neutron) with "I"3 = −. It was later noted that the isospin projections were related to the up and down quark content of particles by the relation:<br>