* If the net external torque on a system is zero, then both the magnitude and direction of the of a rotating system remains constant. If t_net = 0 then L = constant.

* Any force whose line of action passes through the axis of rotation of a system (Example) cannot create a torque on the system and cannot change the system's angular momentum.

* A rotating system can change its state of rotation without have any external agent acting on it if the system can change its moment of inertia by rearranging its distribution of mass internally. A spinning ice-skater who pulls in their arms is a classic example of an internal change in the moment of inertia.