Heisenberg noted that the mathematical formulation of this symmetry was in certain respects similar to the mathematical formulation of non-relativistic spin, whence the name "isospin" derives. The neutron and the proton are assigned to the doublet (the spin- 1 ⁄ 2, 2, or fundamental representation) of SU(2), with the proton and neutron being then associated with different isospin projections I 3 = + + 1 ⁄ 2 and − + 1 ⁄ 2 respectively. The pions are assigned to the triplet (the spin-1, 3, or adjoint representation) of SU(2). Though there is a difference from the theory of spin: The group action does not preserve flavor (in fact, the group action is specifically an exchange of flavour).