The letters n, I and m proposed by Bohr, Sommerfeld and Zeeman respectively for quantisation of angular momentum in classical physics were later on obtained as the results of solution of Schrödinger wave equation based on quantum mechanics. The term n, $\mathcal{l}$, m were named as principal quantum number, azimuthal quantum number and magnetic quantum number respectively. The fourth quantum number i.e., spin quantum number, s was given by Uhlenback on the basis of two spins of electrons. The first two quantum numbers also decides the nodes of an orbital.