  Everett's original formulation left the exact meaning of these relative states and their relations to observers’ experience and beliefs open, and there have been different Everett-like interpretations of these states. Probably the most popular reading of Everett is the splitting-worlds interpretation (see DeWitt 1971, Everett's relative-state formulation of quantum mechanics, Barrett 1999, and references therein). In the splitting-worlds interpretation, each of the branches of the state |ψ11&gt; refers to a different class of worlds (all of which are real) where the states of the L-apparatus, R-apparatus and the particles are all separable: Class-1 worlds in which the L-particle is in the state z-spin ‘up,’ the R-particle is in the state z-spin ‘down,’ the L-apparatus is in the state of pointing to the outcome z-spin ‘up’ and the R-apparatus in the state of being ready to measure z-spin; and class-2 worlds in which the L-particle is in the state z-spin ‘down,’ the R-particle is in the state z-spin ‘up,’ the L-apparatus is in the state of pointing to the outcome z-spin ‘down’ and the R-apparatus is in the state of being ready to measure z-spin. More generally, each term in state of the universe, as represented in a certain preferred basis, reflects the states of its systems in some class of worlds; where the range of the different classes of worlds increases whenever the number of the terms in the quantum state (in the preferred basis) increases (this process is called ‘splitting’).