A fermion is a particle with a half-odd integer spin (, , etc.). All fermions follow the Fermi-Dirac statistics and obey the Pauli exclusion principle. Fermions include all quarks, leptons, and all composite particles made up of an odd number of quarks, leptons such as all baryons and many atoms and nuclei. Additionally all the anti-particles of the quarks and leptons are considered fermions as well.