  A criterion to locate tricritical points in phase diagrams is proposed. The
criterion is formulated in the framework of the Elementary Catastrophe Theory
and encompasses all the existing criteria in that it applies to systems
described by a generally non symmetric free energy which can depend on one or
more order parameters. We show that a tricritical point is given whenever the
free energy is not 4-determined. An application to smectic-C liquid crystals is
briefly discussed.
