  We introduce the functor * which assigns to every metric space X its
symmetric join *X. As a set, *X is a union of intervals connecting ordered
pairs of points in X. Topologically, *X is a natural quotient of the usual join
of X with itself. We define an Isom(X)-invariant metric d* on *X.
  Classical concepts known for H^n and negatively curved manifolds are defined
in a precise way for any hyperbolic complex X, for example for a Cayley graph
of a Gromov hyperbolic group. We define a double difference, a cross-ratio and
horofunctions in the compactification X-bar= X union bdry X. They are
continuous, Isom(X)-invariant, and satisfy sharp identities. We characterize
the translation length of a hyperbolic isometry g in Isom(X).
  For any hyperbolic complex X, the symmetric join *X-bar of X-bar and the
(generalized) metric d* on it are defined. The geodesic flow space F(X) arises
as a part of *X-bar. (F(X),d*) is an analogue of (the total space of) the unit
tangent bundle on a simply connected negatively curved manifold. This flow
space is defined for any hyperbolic complex X and has sharp properties. We also
give a construction of the asymmetric join X*Y of two metric spaces.
  These concepts are canonical, i.e. functorial in X, and involve no
`quasi'-language. Applications and relation to the Borel conjecture and others
are discussed.
