[...] various phonetic usages and various written usages can be ordered to the expression system of one and the same linguistic schema. A language can suffer a change of purely phonetic nature without having the expression system of the linguistic schema affected, and similarly it can suffer a change of a purely semantic nature without having the content system affected. Only in this way it is possible to distinguish between ‘phonetic shifts’ and ‘semantic shifts’ on the one hand, and ‘formal shifts’ on the other.
- Hjelmslev (1961), a pag.105