  We discuss various bifurcation problems in which two isolated periodic orbits
exchange periodic ``bridge'' orbit(s) between two successive bifurcations. We
propose normal forms which locally describe the corresponding fixed point
scenarios on the Poincar\'e surface of section. Uniform approximations for the
density of states for an integrable Hamiltonian system with two degrees of
freedom are derived and successfully reproduce the numerical quantum-mechanical
results.
