Fermions, e.g. leptons (including electrons), quarks, baryons and many atoms and nuclei
The principle is an understanding that no two, or more, identical fermions (particles with half-integer spin) with the same direction of spin can occupy the same quantum state within a quantum system. Very broadly, and generally, it says that two or more identical pieces of matter cannot be in the same place at the same time. The Pauli Exclusion Principle applies to electrons but is not limited to them. Photons do not obey the Pauli exclusion principle.
