> if /th/ is so unstable, why does it exist at all? You misunderstand me. I am saying that random, irregular change is unlikely to create the sound /th/ (or /θ

> if /th/ is so unstable, why does it exist at all? You misunderstand me. I am saying that random, irregular change is unlikely to create the sound /th/ (or /θ/ in phonetic notation, which I’ll use from now on). As you correctly note, by itself that means very little, but it does reinforce all the other evidence I and others have mentioned. (The sound in the word ‘brother’ was formed via a different process: it is the product of regular sound change, specifically Grimm’s Law [https://en.wikipedia.org/wiki/Grimm's_law] https://en.wikipedia.org/wiki/Grimm's_law]. This converted all voiceless stop sounds to fricatives: if /p/→/f/ and /k/→/x/, it makes sense that /t/→/θ/ too, because that follows the same pattern.)