A teacher I had several years ago said that the whole Treiglad Llais was dying out, to his sorrow.
Place names mutate after other causes.
Dw i’n byw yng Nghefneithin. I live in Cefneithin.
If I was working for Radio Cymru, I suspect their rule book, style book, would include TL after a/â. It gets more complicated with foreign place names.
I actually agree with him. I think it sounds lovely when those letters ‘breathe’!
Here’s @garethrking on aspirate mutation, on another forum:
http://www.forumwales.com/fwforum/viewtopic.php?p=75968#p75968
Why does everyone think I don’t like the treiglad llaes?? I love the treiglad llaes, and you’ll see it all over the place in my books. Duwcs, I even say “te a choffi” myself! And “cael a chael”. And “Lloegr a Chymru”. Yup… I would say all these, and I would expect a high proportion of Welsh speakers to do the same.
By the same token however, I would not say “jin a thonic”, or “bws a thacsi”, except as a joke, and I’ve heard plenty of Welsh speakers do these as jokes, believe me.C to CH is consistently applied in various circumstances; P to PH and T to TH are heard much less frequently, and often sound stilted (see thonic above) - you do get them on the front of verbs-with endings, but very inconsistently…many speakers use SM in these circumstances, as someone else here has pointed out correctly elsewhere.
Look what the Modern Welsh Dictionary (p. xiii) says about this:
“…only the change C to CH is applied with any consistency in the modern spoken language, though all three are the norm in writing.”
I agree with every word of that statement. 8)
They do it consistently for C > CH, very very erratically for P > PH and T > TH. Mam a thad and sounds fine for this reason - whereas bws a thacsi sounds (frankly) daft, or even worse pretentious. Another nice set phrase, this one with P > PH, is a phelly and suchlike, which is a contraction of a phethau felly and such things.
Now, similarly with placenames - C > CH, normal, other two not: a Chaerdydd and Cardiff sounds normal and correct, while a Thalybont and Talybont sounds faintly precious to ordinary people I think.
I think T > TH is the least natural of the three overall.
Got into dreadful trouble with the language police for saying all this years ago in the grammar, but it is the truth, and anyway I don’t care. 
Personal names are generally immune to mutation, by the way.
Diolch yn fawr iawn Gareth and for all the other replies. I never realised I was touching on something that has a little bit of controversy attached to it - nice tobe controversial sometimes.
Oh yes - I remember that!
While we are on the whole mutation topic - do you mutate the names of your countries if you live outside Prydain? (Belarus, for example:)
I know that grammatically you should, but I’ve heard people drop the mutation sometimes, and I was interested in how comfortable people feel with that.
I tend to do it with most place names. I think I heard someone say that for place names outside of Wales it’s optional, but I can’t remember for sure.
“Wyt ti’n dod o Felarws? Neu wyt ti’n byw ym Melarws?”
I’m not sure this has a ‘quick answer’ but what do you all think of the following article.
Mihil
That is correct. I’ve heard native speakers say dw i’n mynd i Firmingham I’m going to Birmingham, for example…although I do urge people here NOT to employ the same principle if they’re going to Buckingham Palace.
With non-Welsh placenames beginning with C, practically anything goes:
Mae hi’n byw yn Camden
Mae hin’n byw yng Nghamden
Mae hi’n byw yn Gamden
and
Mae hi’n dod o Camden
Mae hi’n dod o Gamden
all fine really.
Plenty of native speakers even use SM with placenames after yn in :
Mae hi’n byw yn Fangor
Mae hi’n byw yn Dregaron
This is just retreat of NM as SM is generalised. You’d lose marks for it in a written exam, though… 
Oh, I adore the idea of *uckingham… 
Just a quick question on “banc”, particularly the title of the song Danybanc, featured in the Welsh Music thread. In Wales, can banc mean an incline (as well as a river bank)? I ask this because I am used to “Bank Foot” meaning the bottom of a hill, but I think that in England, this only applies in certain regions. Is it wishful thinking on my part for Danybanc to fit this meaning rather than the “below the river bank”, which would be strange?
Danybanc - probably means below the bank as in a slope/hillock type bank/hill ?
Troed y rhiw for example means foot of the hill…same essence
Quick question/ Cwestiwn cyflym: In what context can one use “yw bod”…or is it not used in speech?
I’ve seen it written a fair few times.
Example:
Y broblem fwya yw bod y lleill ddim yn deall
The biggest problem IS THAT the others don’t understand
Used in speech all the time.
Diolch…can you use “yw mai” / “yw taw”…at all…Ive made this up in my head…because I have never heard it yet! 
Yes you can - example:
Y broblem fwya yw mai ni sy’n gyfrifol am hyn oll
The biggest problem IS THAT it is we who are responsible for all this.
Sorry, Gareth, but I am being ultra thick. ‘taw’??? My mind has gone blank on this!