The occurrence of electronic shells and supershells in sodium clusters can be understood in a rather simple way by invoking Bohr's quantum postulate from 1913 and the Fermi gas expression for the relation between density and Fermi momentum, which in turn follows from the Pauli principle. An analysis of theoretical and experimental data on this basis lets the closed electronic orbits of triangular and square shape emerge as the main contributors to the supershells, in agreement with the idealized predictions by Balian and Bloch (1971).