As an example, consider the measurement model in subsection 7.2, where in the family \(\FC_u\) at time \(t_2\) the measurement apparatus is in a superposition state corresponding to a projector which is incompatible with the projectors representing different pointer positions, which are employed in the family \(\FC_1\). The histories approach says the physicist is at Liberty to employ either family, both of which are valid ways of describing the quantum world, but Incompatibility forbids combining them into a single description. The prohibition on combining families is based on the fact that certain quantum projectors do not commute, something quite specific to quantum mechanics with no classical counterpart. In classical mechanics the true state of affairs for a physical system is always represented by a single point in the phase space. It is this feature that has to be abandoned in quantum mechanics interpreted using histories. See the  Further Reading supplement for additional details. Griffiths (2014).)--> 