  A buckled sheet offers a reservoir of material that can be unfurled at a
later time. For sufficiently thin yet stiff materials, this geometric process
has a striking mechanical feature: when the slack runs out, the material locks
to further extension. Here we establish a simple route to a tunable locking
material - a system with an interval where it is freely deformable under a
given deformation mode, and where the endpoints of this interval can be changed
continuously over a wide range. We demonstrate this type of mechanical response
in a thin cylindrical shell subjected to axial twist and compression, and we
rationalize our results with a simple geometric model.
