In many industries the tariffs are not strictly proportional to the
quantity purchased, i. e, they are nonlinear. Examples of nonlinear
tariffs include railroad and electricity schedules and rental rates for
durable goods and space. The major justification for the nonlinear
pricing is the existence of private information on the side of
consumers. In the early papers on the subject, private information was
captured either by assuming a finite number of types (e. g. Adams and
Yellen, 1976) or by a unidimensional continuum of types (Mussa and
Rosen, 1978). Economics of the unidimen- sional problems is by now well
understood. The unidimensional models, however, do not cover all the
situations of practical interest. Indeed, often the nonlinear tariffs
specify the payment as a function of a variety of characteristics. For
example, railroad tariffs spec- ify charges based on weight, volume, and
distance of each shipment. Dif- ferent customers may value each of these
characteristics differently, hence the customer's type will not in
general be captured by a unidimensional characteristic and a problem of
multidimensional screening arises. In such models the consumer's private
information (her type) is captured by an m-dimensional vector, while the
good produced by the monopolist has n quality dimensions.