We are now ready to oppose systematically grammaticalization and degrammaticalization. The following diagram summarizes the various aspects of grammaticalization seen in preceding chapters. It shows them as properties and processes which change from left to right.

The properties and processes taken together characterize grammaticalization and coincide in the typical cases. Their inversion, i.e. processes that run from right to left, and properties that change from right to left, in the diagram are proper of degrammaticalization. Processes of the former type occur every day in all languages of the world. They are essentially uncontrolled, happening almost “by themselves”. Processes of the latter type are exceedingly rare. They depend on the creativity of certain speakers who initiate an unusual change and on the receptivity of their peer group to take it up. It seems that a theory of language has to allow for this kind of exceptions.