However, one can consider properties of a spin operator Ŝ alone. It has three Cartesian components and transforms under rotations as a vector. Operators involving spin may obey other transformation rules. For example, the Ŝ2 operator, formed as a dot product of the two spin operators, is a scalar operator: it has only one component and it does not change upon the rotation of the coordinate frame. The spin-orbit operator of the BP Hamiltonian is