The Zh in Breizh, Brezhoneg and Breton Place Names

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Something that always puzzled me about Breton was this 'z/zh'. It is obviously a sound shift which features only in modern Breton (not Old Breton - the one we need to use for place names), but where does it come from?

The 'zh' spelling was introduced in 1941 to merge the representation of different dialect sounds: it represents /h/ in Vannetais and /js/ in the Trégorrois, Cornouallais amd Léonard dialects.

Dialects of Brittany

Let's look at the Breton name for Brittany first - Breizh, which is pronounced 'brays' [/brejs/].

A l'Aise Breizh

It comes from a common Brittonic Prythain which can be found in Old Breton [Preden or Prydain], Welsh [(Ynys) Prydain] and Cornish [Preden]. 
In Latin this is Britannia and becomes, in French, Bretagne. In the old days the two Britains were Britannia Major (Great Britain) and Britannia Minor (Brittany/Little Britain/Moindre Bretagne). See here for an Irish language perspective.

Little Britain - another name for Brittany

'zh' also appears in the name for the Breton language which is Brython in Welsh and brezhoneg in Breton . 

THE BRETON LANGUAGE INSTITUTE

It is here that  a pattern starts to emerge. 

The general rule is that single voiceless plosives (e.g. t, tt, Ɵ) and voiced plosives (e.g. d, dd, ð) become soft spirants (e.g. z, zh, v) in intervocalic positions (between vowels).

RENNES (French), ROAZHON (Breton)

Here are a few place name examples:

Rennes (Raozhon in Breton) ← Redones/Riedones (Gaulish/Latin).
Arzon (Arzhon in Breton) ← aredunum (Old Breton).
Beuzec (Beuzeg in Breton) ← Budoc (Old Breton).
Dinard (Dinarzh in Breton) ← Dinard (Old Breton).
Limerzel (Limerzher in Breton) ← Eccelesia Martyrum (Latin).
Tredaniel (Trezaniel in Breton) ← Tre-Daniel (Old Breton)