  We analyse the evolution of a sigmoidal (S shaped) active region toward
eruption, which includes a coronal mass ejection (CME) but leaves part of the
filament in place. The X-ray sigmoid is found to trace out three different
magnetic topologies in succession: a highly sheared arcade of coronal loops in
its long-lived phase, a bald-patch separatrix surface (BPSS) in the hours
before the CME, and the first flare loops in its major transient intensity
enhancement. The coronal evolution is driven by photospheric changes which
involve the convergence and cancellation of flux elements under the sigmoid and
filament. The data yield unambiguous evidence for the existence of a BPSS, and
hence a flux rope, in the corona prior to the onset of the CME.
