The spin-orbit two-body interaction has been used for a long time in nuclear structure studies and is unique. In contrast, two different expressions are used in nuclear scattering studies. One of them is stronger when the eigenvalues of $(\ensuremath{\ell}\ifmmode\cdot\else\textperiodcentered\fi{}\ensuremath{\sigma})$ of the two particles involved are opposite to each other, such as the interaction used in nuclear structure studies for an even parity excitation. This expression of the spin-orbit interaction can be qualified as ``normal'' in view of the similar behavior in these two applications. The other expression of the spin-orbit interaction involves the sum of these two eigenvalues.