While diglossia as a fact of linguistic culture may be stable, the distribution of domains reserved for one variety or other can vary; the dominance of a particular domain by a particular variety can shift, with one variety encroaching on domains previously restricted to another. In Tamil, for example, the political speech was once restricted to the domain of the H variety, but nowadays political speeches only begin and end in H; in between, L variety predominates (probably as a mark of solidarity). In journalism, especially in political cartoons, etc. one also sees a shift from H to L in many linguistic cultures. In Alemannic Switzerland and some other linguistic cultures, the development of television has opened up a domain that has become almost exclusively that of the L variety, especially in `live' interviews, talk shows, game shows, sports reporting, etc. where use of H would seem stilted and unnatural.
On the other hand, social forces within a particular linguistic culture can move to eliminate diglossia, as was the case when medieval Latin was displaced during the Renaissance by various European vernacular languages; diglossia is giving way in present-day Greece, where it had held sway until a government decree ordained the shift from H (katharevousa) to L ( demotiki) in many domains Diglossia was more extreme in pre-modern Bengali and Telugu than it is today, as a result of movements led by prestigious writers (Tagore for Bengali) to democratize access to literacy and education, and modernize their languages. Latin held on in German linguistic culture until the early 18th century in a number of restricted domains (scholarly writing, university lectures). When and if diglossia is more or less eliminated, or made less extreme, by the choice of a more modern colloquial norm we would have to, by rights, speak of a kind of language shift. To ignore shift when it takes place within a diglossic continuum would be to perpetuate the notion that diglossia is in effect irrelevant.