  We study open books on three manifolds which are compatible with an
overtwisted contact structure. We show that the existence of certain arcs,
called sobering arcs, is a sufficient condition for an open book to be
overtwisted, and is necessary up to stabilization by positive Hopf-bands. Using
these techniques we prove that some open books arising as the boundary of
symplectic configurations are overtwisted, answering a question of Gay in
Algebr. Geom. Topol. 3 (2003) 569--586.
