Since the importance of the monopole interaction was first emphasized in 1960s, roles of monopole strengths of two-body nucleon–nucleon interaction in shell structure have been discussed. Through the monopole strengths, we study the roles in shell-structure evolution, starting from explicit forms of the interaction. For the tensor component of the interaction, we show the derivation of the relation, (2j>+1)Vjj>+(2j<+1)Vjj<=0, with a detailed manipulation. We show that one-body spin–orbit term appears in the multipole expansion of two-body spin–orbit interaction. Only the spin–orbit components can affect the spin–orbit energy splitting between spin–orbit partners, when the spin–orbit partner orbits are fully occupied.