  In this paper we provide a characterisation of rational developable surfaces
in terms of the blossoms of the bounding curves and three rational functions
$\Lambda$, $M$, $\nu$. Properties of developable surfaces are revised in this
framework. In particular, a closed algebraic formula for the edge of regression
of the surface is obtained in terms of the functions $\Lambda$, $M$, $\nu$,
which are closely related to the ones that appear in the standard decomposition
of the derivative of the parametrisation of one of the bounding curves in terms
of the director vector of the rulings and its derivative. It is also shown that
all rational developable surfaces can be described as the set of developable
surfaces which can be constructed with a constant $\Lambda$, $M$, $\nu$ . The
results are readily extended to rational spline developable surfaces.
