  In order to identify the basic conditions for thermal rectification we
investigate a simple model with non-uniform, graded mass distribution. The
existence of thermal rectification is theoretically predicted and numerically
confirmed, suggesting that thermal rectification is a typical occurrence in
graded systems, which are likely to be natural candidates for the actual
fabrication of thermal diodes. In view of practical implications, the
dependence of rectification on the asymmetry and system's size is studied.
