  A gravitational field can be seen as the anholonomy of the tetrad fields.
This is more explicit in the teleparallel approach, in which the gravitational
field-strength is the torsion of the ensuing Weitzenboeck connection. In a
tetrad frame, that torsion is just the anholonomy of that frame. The infinitely
many tetrad fields taking the Lorentz metric into a given Riemannian metric
differ by point-dependent Lorentz transformations. Inertial frames constitute a
smaller infinity of them, differing by fixed-point Lorentz transformations.
Holonomic tetrads take the Lorentz metric into itself, and correspond to
Minkowski flat spacetime. An accelerated frame is necessarily anholonomic and
sees the electromagnetic field strength with an additional term.
