Diglossia is the situation where different spheres of social interaction in a community use forms of language which are not merely different registers, with some tweaking, but different linguistic systems completely. There is a High language form, used in writing, formal contexts, literature, education, officialdom, the media; and there is a Low language form, used in speaking, the home, the marketplace. If it's written down at all, it's as a transcription, or deviance: it's not a norm, or something to emulate. Usually the two variants are linguistically related; but they're not closely related enough to be merely different registers, as they are in English. They're at least dialects apart.
That describes a lot of linguistic situations in the world. Classical Arabic vs. Colloquial Arabics. High German vs. Swiss German. Most creoles, with the colonial language at one end (the acrolect), and the creole at the other (the basilect). Because the high and low variants are usually linguistically related, there can be a spectrum of variation, rather than two well-defined extremes, and people can play with how high or low they are being.
But the thing about diglossia is, it is a stable arrangement, in which people know which form to use where; where using the wrong form is nonsensical, it's ludicrous. There's been a parallel in Greek for the past hundred years, but it hasn't been Puristic vs. Demotic. It's been Standard Greek vs. Cypriot. Giving a lecture in Cypriot, or a speech, or having a news article in Cypriot, is unthinkable, it's nonsense (though the speaker might pop in a dialect word for colouring). But that does not mean Cypriots think their dialect is bad and not worth speaking, even if they occasional say it is. If you speak standard Greek to many a Cypriot, you may be excused as a Greece Greek, a "penpusher" (καλαμαράς, because none but a penpusher would speak standard Greek). If you're not a penpusher by birth, then you're a penpusher by affectation, and this engenders hostility. Not everywhere and and all times in Cyprus, but still often enough that the dialect is quite healthy.
Greece in the 20th century was not like that, and it's hard for contemporary Greeks to appreciate that Greece in the 19th century *was* like that. In the 19th century, the High language was Puristic: it may have been incoherent, it may have been artificial, it may have been unworkable, but noone in 1860 Athens disputed that it was the appropriate language for literature, education, officialdom, or the media. Likewise, there was a Low language, which were the various dialects of the vernacular. It appears that a vernacular Koine was already coalescing around Peloponnesian (and under significant Puristic influence) in Athens, but it was neither stabilised yet, nor was anyone trying to stabilise it. In fact, Chatzidakis, who ran Greek linguistics for decades, was denying there was anything like a Demotic Koine several decades after there clearly was.
Something switched by 1920: by 1920, almost all serious literature was being written in the vernacular, educationalists were starting to advocate at least beginning schooling in the vernacular, there was an attempt to translate the Gospels into the vernacular (which met with riots by university students, and a few deaths), as well as Aeschylus (more riots, this time toppling a government, and closing the National Theatre for the next thirty years). Officialdom did not adopt the vernacular until 1976; and even then, the vernacular they adopted was not the vernacular the early advocates had hoped for. But even by the 1950s, Puristic was a frequent target of derision; and derision is not part of the deal with the High language form in a diglossia. The whole point of a diglossia is that the High variant is... High.
Nor is politicisation part of the deal with diglossia. By the 1950s, with the aftermath of the Greek Civil War, you could tell someone's political orientation by how they declined nouns in -ις, -εως. (Some of you may have seen this in Browning's Mediaeval and Modern Greek, and thought it was an exaggeration. It was not.) The 1980s still saw an echo of this, with the Socialists in government using folksy morphology in their just-as-wooden political language; and the Communists had long turned the vernacular meter up to eleven.
The Marxist historian Kordatos is the only instance I've seen in print of the future particle θαν, which could be transitional between θενα and θα. It could also be analogy with any number of particles with nu movable (/n/ as liaison), and that's an analogy joined by ναν for να. As usual in such cases, it's probably both.
So diglossia in the 20th century had degenerated into a conflict, the Greek Language Question, in which the primacy of Puristic was disputed along party lines. Situations where society is linguistically stratified are not always clean. Italy's Language Question involves such subtle shading between dialect and standard, a linguist has decided to call it there dilalia, and Norway's situation is more split on region than it is on ideology, and hardly at all on social register. But that just means 20th century Greece is a spectacularly bad example of diglossia, and it doesn't tell you how diglossia is meant to work, like contemporary Egypt or Haiti does. Or Cyprus.
It also makes it difficult for contemporaries to picture the world before 1920—especially before 1875—when Greek diglossia was a lot closer to what is happening in Egypt and Cyprus now. It's hard for us now to believe, as Peter Mackridge wrote in a paper once, that the Communists in the 1910s dismissed the advocacy of the vernacular (Demoticism) as a bourgeois distraction, while Head Demoticist Psichari was a royalist.
I think it was: Mackridge, P. 1990. Katharevousa (c.1800–1974): An Obituary for an Official Language. In Sarafis, M. & Martin, E. (eds), Background to Contemporary Greece. London: Merlin Press. 25–51.
It's just as hard for us to believe that in the 1840s, Puristic was a tool of modernisation, and was a means to integrate Greece into Western Europe. Well, the Western Europe thing is easy to see; it's the modernisation though millenium-old datives and infinitives that's hard to get. It's hard to believe that people portrayed the vernacular, not merely as uncouth, but as the language of Ottoman servility. Which is why Psichari, in his 1888 manifesto My Voyage (about his first field-trip to Greece) made a point of citing Sapphic Odes in Puristic in honour of Sultan Abdul Hamid. [Link to First Edition, before he whitewashed out all the Constantinopolitan dialect.]
Psichari would have you believe that the switch in the status of Greek diglossia, with conflict over the legitimacy of Puristic, was his doing; and the standard histories of Greek language and literature will tell you the same. Psichari—who after all coined the term "diglossia"—made an extremely influential critique of Puristic, it's true, and his critique of the incoherence and arbitrariness of Puristic has stood the test of time (with the added benefit of being hilarious.)
Yet the undermining of Puristic did not start in 1888. What started undermining it was that the Ionian islands, which were British until 1864, never got on board with Puristic. As a result, Demotic literature from the Ionian islands enjoyed prestige—not least Solomos', already revered as the national poet, though some critics did grumble at his folksiness. The first chip in the Athenian edifice of Puristic literature was when the the Academy of Athens allowed the Ionian Islander Valaoritis, in the 1870s, to recite his vernacular verses on Greek National Day.
The authors who wrote Demotic in the 1890s and 1900s grouped around Psichari and respected him—and they inevitably disappointed him, as their language compromised with Puristic in a way he never accepted. But they did not come out of nowhre, he did not conjure them into being himself.
Psichari was a better linguist than an author, and a better historical linguist than a sociolinguist. He approached Demotic with Neogrammarian rigour, proclaiming Ausnahmlosigkeit as "Language admits no compromises!"
OK, I'll unpack that. Historical linguistics before 1880 couldn't explain all the sound changes of Germanic, and went with a mellow, hippy, "shit happens" attitude to exceptions. In 1875, the last seemingly inexplicable sound change of Germanic was explained; and the Neogrammarians, the Young Grammarians (Junggramatiker) who followed proclaimed it as an article of faith that There Is No Such Thing As Exceptions (Ausnahmlosigkeit): if you can't work out why a change has happened, you're not looking hard enough.
Psichari being a man of his time, and not yet pathologically hating the Germans for killing his son in World War I, he embraced that belief in Ausnahmlosigkeit, and applied it to his Demotic activism. Historical Linguistics worked with normal, predictable language change. Compromise with Puristic would give rise to linguistically odd hybrids, which the Unlettered Folk were clearly having trouble coping with. So compromise was linguistic nonsense.
And if you have a sufficiently narrow view of language, it is nonsense. And "nonsense" is precisely how a typologist has to regard Standard Modern Greek, which has made precisely the kinds of compromise that Psichari dismissed. The phonology of Standard Modern Greek, with its influx of spelling pronunciations from Ancient Greek, is loony tunes: /anðrono/? /asθma/? /efθrafsto/?
The tug of war in -ις -εως between an archaic and a modern declension is similarly absurd, and has all the characteristics of committee design: first archaic singular and plural, then both archaic and modern singular but archaic plural, now modern singular and archaic plural. People laughed at Psichari for having a modern plural in πρότασες "sentences" instead of προτάσεις, switching the third declension to the first—like every other vestige of the third declension has done in the vernacular. But writers in the 17th century vernacular did the same, because they had noone to tell them to compromise with Puristic. And those who laugh forget that ράχες, the Standard Modern Greek for "backs", also started life in the third declension.
So the plural of an Ancient -ις -εως in Standard Modern Greek is not dictated by the normal laws of language change. It is dictated by snobbery: the elite have "sentences", the hoi polloi have "backs". That's absurd.
But it's also part of how language rolls, because Psichari's was much too narrow a view of how language works. Language is also a vehicle for social attitudes, and those social attitudes reflect back on the form of the language. In purely linguistic terms, if Ancient ῥάχεις could become vernacular ράχες, there's no earthly reason why Ancient προτάσεις shouldn't become vernacular πρότασες; and Psichari concluded as much at a time when people were advocating you should still say both ῥάχεις and προτάσεις. But for whatever reason, the burghers of Athens decided that was a bridge too far even in their Demotic: ράχες is fine, πρότασες is extremist. Because a real language, spoken in real social contexts, does admit compromise: Puristic could not just be wished away in a puff of smoke. (Just as, for that matter, there aren't any pure languages, and the Neogrammarians' contemporaries knew the family tree of languages was a distortion.)
And Puristic has worked its influence on a Modern Greek's linguistic intuition so thoroughly, they can no longer see the absurdities Puristic has imposed on their language. Which makes me dispute Motorcycle Boy's conclusion from a few posts ago: people *can* speak an artificial language, and not realise it. In some way, after all, any codified literary language is artificial.
The influence of Puristic is pervasive enough to illustrate in the following anecdote. To set the context: the Greek Army was an institution well placed to roll out Puristic to the populace: you had a captive audience, that you barked orders to, that they had to obey. It was the one place where you could convince people that the word for "fire" is not φωτιά "lightness" (or λαμπρόν "bright" in Cyprus, or στιά "hearth" in the Ionian islands), but the Ancient πῦρ.
Psichari of course had a field day with this: the sergeant could bark "fttpt" or "herring", and the soldier will still shoot; that doesn't mean you've rewired his brain to call "fire" anything but φωτιά (or λαμπρόν or στιά).
As it turns out, my brain has been rewired. Not quite in the way Psichari said, but close.
When King Otto arrived in Greece in 1833, an honour guard of veterans was set up to fire off a 21 gun salute. When the appointed time came, the designated officer walked up, and proudly shouted, in the only form of Greek worthy of the occasion:
OFFICER #1: ... Ignis! [Πῦρ!]
OFFICER #1: ... Ignis! [Πῦρ!]
OFFICER #1: ... Ignis? [Πῦρ;]
VETERANS: ... (Who the hell's this Innis guy he keeps calling out for?) (Nay, nay, you see, he's addressing his Majesty in his native Barvarian.)
OFFICER #2 (BILINGUAL IN ANCIENT AND MODERN GREEK): [from the crowd] ... *FIRE*, damn your hides! [Φωτιά, πανάθεμά σας!]
VETERANS: ... Oh! *bang bang bang* (See, told you! That's Sgt Innis right there.)
When I read this, I thought to myself (in Greek): what does setting things on fire (φωτιά) have to do with shooting guns (πυρά)?
Then I translated both words into English.
Then I was sore amused.
There's a simple metaphor in many a language between setting things on fire and shooting guns. Hence, gunfire, and fire!. Saying fire! in Ancient Greek at the barracks did not succeed in reviving the ancient Greek word for setting things on fire.
But it did succeed in destroying the metaphoric link: the Ancient Greek word for "fire" is the only word now used for "fire" in a military context—that is, gunfire. The Modern Greek word for "fire" is the only word now used for "fire" in any other context. And modern speakers do a double-take, to realise that gunfire has something to do with burning.
Not what people in 1833 had in mind...