  Quad-surfaces are polyhedral surfaces with quadrilateral faces and the
combinatorics of the square grid. A generic quad-surface is rigid. T-hedra is a
class of flexible quad-surfaces introduced by Graf and Sauer in 1931.
Particular examples of T-hedra are the celebrated Miura fold, discrete surfaces
of revolution, and discrete molding surfaces. We provide an explicit
parametrization of the isometric deformation of a T-hedron. T-hedra have a
smooth analog, T-surfaces. For these we provide a synthetic and an analytic
description, both similar to the corresponding descriptions of T-hedra. We also
parametrize the isometric deformations of T-surfaces and discuss their
deformability range.
