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

==== Putting it together: ====
* Proto-Celtic develops true geminate sonorants (ll, nn, rr, mm).
* Goidelic (Old Irish): - Geminates lose pure length, but are reanalysed as fortis sonorants (tenser, stronger); written ll, nn, rr vs lenis l, n, r. - In modern Irish, the fortis/lenis contrast mostly collapses, but orthographic doubling (coll, cill, etc.) survives as a fossil.
* Brythonic (Welsh): - Some of those fortis lateral / rhotic lines continue pushing rightwards into devoicing: - lateral → ll [ɬ] - rhotic → rh [r̥] - Welsh keeps ll, rh as its own voiceless sonorant phonemes.

So in exactly the sense you care about:

: 

coll ~ cyll/collen and cill ~ cell are the cleanest modern visible pairs; toll ~ twll shows the same Proto-Celtic tull- path once you look one level deeper, even though Irish no longer writes the double.