Isospin multiplet

In particle physics, isospin multiplets are families of hadrons with approximately equal masses. All particles within a multiplet, have the same spin, parity, and baryon numbers, but differ in electric charges. An example of an isospin multiplet is the nucleon multiplet consisting of the proton and the neutron.

The existence of the multiplets with approximately equal masses owes to the fact that the masses of up and down quarks are approximately equal (compared to a typical hadron mass), and the strong interaction is quark flavour blind. This makes the isospin symmetry a good approximation.