On the structure of mu-classes

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We prove that, if μ< n/2, then every rational plane curve of degree n and class μis a limit of parametrizations of the same degree and class μ+1. This property was conjectured in D.Cox, T.Sederberg,F.Chen's paper: "The moving line ideal basis of planar rational curves" (Comput. Aided Geom. Des. 15 (1998), 803-827), and its validity allows an explicit description of the variety of parametrizations of degree n and class μ, for all (n,μ).