  We consider developable surfaces along the singular set of a swallowtail
which are considered to be flat approximations of the swallowtail. For the
study of singularities of such developable surfaces, we introduce the notion of
Darboux frames along swallowtails and invariants. As a by-product, we give a
new example of a frontal which is locally homeomorphic to a swallowtail.
