Liouville structures
  A 'Liouville structure' is a structure isomorphic to a cotangent vector fibration. A Liouville structure is an essential ingredient of every variational formulation of a physical theory. For reasons of interpretation the Liouville structure can not be replaced by the corresponding cotangent fibration. We give a precise definition of Liouville structures, study their properties, and give examples used in variational formulations of mechanics. 
W. M. Tulczyjew, P. Urbanski