  We study the geometric structure of port-Hamiltonian systems. Starting with
the intuitive understanding that port-Hamiltonian systems are "in between"
certain closed Hamiltonian systems, the geometric structure of port-Hamiltonian
systems must be "in between" the geometric structures of the latter systems.
These are Courant algebroids; and hence the geometric structures should be
related by Courant algebroid morphisms. Using this idea, we propose a
definition of an intrinsic geometric structure and show that it is unique, if
it exists.
