Speaking of marginal things, the Latin vocative is surely the perfect example of a marginal case. The fact that one declension paradigm has a case that is absent in the others is the sort of quirk that it's useful to be aware of when aiming for naturalism.

Pete Bleackley
The Fantastical Devices of Pete The Mad Scientist -

-----Original Message-----
From: R A Brown <[log in to unmask]>
To: [log in to unmask]
Sent: Sun, 09 Apr 2017 2:34 p.m.
Subject: Re: Usage: Church Latin vocatives

On 09/04/2017 12:18, And Rosta wrote:
> On 9 Apr 2017 08:24, "R A Brown" wrote:
>> The regular vocative singular was _Dee_ which was used by
>> Tertullian and Prudentius. But pronouncing this as two
>> syllables (both Es being short) correctly in normal speech
>> is clearly awkward.
>> It was IMO fairly 'natural' for the anonymous
>> translators of the Old latin versions to use _deus_ rather
>> than the phonetically awkward _dee_.
> I am away from my copy of Vox Latina, so let me ask more about this
> awkwardness. Any awkwardness surely can't have been literally purely
> phonetic.

It's awkward in English, isn't it?  I can't think of any 
words where /ɛ.ɛ/ occur in contiguous syllables.  But if 
they do they surely will not retain separate pronunciations 
in allegro speech.

> Assuming that contiguous heterosyllabic (what's the evidence for
> the heterosyllabicity?) identical nonhigh vowels were phonologically licit,
> perhaps the problem was the combination of their rarity and the marginality
> of their phonetic contrast with other long monophthongs?

Fairly marginal and clearly contracted in normal speech to a 
single vowel, cf. cohors ~ cors.  It's clear from the 
Romance languages that the contracted short vowel was the 
norm in speech, e.g. cover [verb] <-- Old French _covrir_ 
(Modern _couvrir_) <-- Vulgar Latin *coperīre ( = Classical 

> Or maybe they were
> phonologically licit in earlier Latin but became illicit along with other
> reorganizations to the vowel system? --

They were marginal and simplified in spoken Latin. 
Turtullian and Prudentius may have used the "correct" _Dee_ 
in writing.  But had this form caught on it would surely 
have been pronounced just as /dɛ/.  The evidence is clear 
that Latin speaking Jews and, later, Latin speaking 
Christians felt happier with _Deus_ /'dɛʊs/