Editing Openai/69186d20-2d14-8013-b9c6-c09000f173f6 (section)

=== Assistant: 2511141729ET ===
251114_1729ET

right, that’s a good refinement, and you’re right to insist on it.

When I said “shared predisposition,” I was compressing two layers that you’re now teasing apart:
# A concrete inherited feature in Proto-Celtic / early Common Celtic
# Branch-specific exploitations of that feature (Goidelic vs Brythonic)

For the ll / fortis vs voiceless story, we really have:

===== Proto-Celtic / early Common Celtic very plausibly had: =====
* real geminate sonorants: ll, nn, rr, mm as distinct segment types, not just “any language could have beefed-up sonorants.”
* those geminates carried morphophonological load, so they were part of the system, not random phonetic noise.

That’s a shared, concrete character state in the ancestor, not just “these languages are the sort that might develop sonorant fortition someday.”

So for e.g. ''PCelt ''koslos → coll / cyll*:
'' There really was a ''single PCelt ''ll'' in that lexical item.
'' Both Goidelic coll and Welsh cyll are reflexes of that ''ll*.

That’s exactly the “diverging reflexes of a shared historical feature” you’re pointing to.

===== Then the branches do different things to that same inherited ''ll'': =====
* Goidelic: - Geminate ll loses pure length, reanalysed as fortis voiced lateral vs lenis /l/. - In modern Irish the fortis/lenis contrast mostly collapses; orthography keeps ll (coll, cill, etc.), phonetically just [l̪ˠ]/[lʲ].
* Brythonic (Welsh): - The same ll line continues to fortify → devoicing in certain contexts. - That becomes phonemic [ɬ], with orthographic ll now dedicated to the voiceless lateral.

So synchronically:
* Irish coll /kɔl̪ˠ/ and Welsh cyll /kɨɬ/ look radically different,
'' but diachronically they are different reflexes of the same PCelt geminate lateral ''ll*.

This is stronger than “shared predisposition”: it’s a shared inherited feature with different branch-internal outcomes.

===== The “shared predisposition” idea is still useful, but at a slightly more abstract layer: =====
* Proto-Celtic’s willingness to use sonorant fortition/gemination morphologically → gives descendants a kind of structural affordance: “sonorants are a place you can encode grammar.”
* Goidelic uses that to keep a fortis/lenis sonorant system for a while, then mostly lets it die.
* Brythonic uses it to push some of those fortis sonorants into voiceless phonemes like [ɬ], [r̥].

So:
* Concrete shared feature: PCelt ll, nn, rr, mm geminates in specific lexemes and morphophonologies.
* Diverging reflexes: Irish orthographic ll vs Welsh ll [ɬ] (and similar cases).
* Shared predisposition: the broader Celtic habit of turning “odd sonorant behaviour” into grammatical material, which both branches exploit in different ways.

You’re absolutely right that in the coll ~ cyll / cill ~ cell type pairs, we’re in the “diverging reflexes of a shared historical feature” regime, not mere convergence. That’s the correct phylogenetic reading.