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

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* Indo-European languages often weaken voiceless stops between vowels, or after vowels.
* Celtic took this quite far: intervocalic /p t k/ → fricatives or approximants, and similar for voiced ones.

Very simplified pathway:
* VCV → V ɸ / θ / x V (or their voiced counterparts)
* Over time: these fricatives sometimes voiced, merged, or disappeared.
* Emotional/conversational weakening after function words (like particles, prepositions, possessives) was very common.

So originally, the difference was purely phonetic:
* …V + C… vs …V + V-C… → consonant sounded “softer” in the latter, because it was in an intervocalic-like environment.

===== By Old Irish (c. 700–900 CE): =====
* Many inflectional endings had been lost or heavily reduced.
* But the consonant alternations remained very salient and systematic: - “soft” vs “hard” versions after certain particles, in certain morphological contexts.
* Speakers generalized this: >

So lenition became:
* Phonological: a contrast between “fortis” stop and “lenis” fricative approximant.
* Morphological: marks gender, case, tense, possession, etc. depending on context (exact pattern fossilized from earlier morphosyntax).

===== All that gets regularized and re-spelled: =====
* Old dot above (ḃ ċ ḋ …) → modern bh ch dh ….
* The triggers (possessives, articles, past particles, etc.) reflect where lost endings and old prepositions once created leniting environments.

So: lenition is fossilized intervocalic/ postvocalic weakening that turned into overt grammar.