 Relative variables are variables whose value does not depend on a single system, but rather on two systems. A well known example is the velocity of an object in classical mechanics. The velocity is always implicitly or explicitly relative to a second object. There is no “velocity of a single object”, irrespectively on any other object, in classical mechanics. Other well-known examples are the electric potential (only the potential of a conductor with respect to another conductor has physical meaning) and position (only position with respect to some other object has physical meaning). Relational quantum mechanics takes a further long step in this direction, assuming that we can make sense of quantum mechanics by assuming that all physical variables are relational in this sense.