  It has been known for more than 300 years that the shape of an inelastic
hanging cable, chain, or rope of uniform linear mass density is the graph of
the hyperbolic cosine, up to scaling and shifting coordinates. But given two
points at which the ends of the cable are attached, how exactly should we scale
and shift the coordinates? Many otherwise excellent expositions of the problem
are a little vague about that. They might for instance give the answer in terms
of the tension at the lowest point, but without explaining how to compute that
tension. Here we discuss how to obtain all necessary parameters. To obtain the
tension at the lowest point, one has to solve a nonlinear equation numerically.
When the two ends of the cable are attached at different heights, a second
nonlinear equation must be solved to determine the location of the lowest
point. When the cable is elastic, think of a thread in a spider's web for
instance, the two equations can no longer be decoupled, but they can be solved
using two-dimensional Newton iteration.
