This Primitive Adûnaic phoneme, represented by the symbol “H” in the typescript of Lowdham’s Report, was a continuant in the k-series (velars), described as a “voiceless back hissing sound, ‘ch’ of Welsh, Gaelic and German (as in ‘acht’)” (SD/417). Very probably it was the voiceless velar fricative which Tolkien often represented with the symbol “χ” (IPA [x]).

In most positions [x] weakened to a “breath h” [h] in Classical Adûnaic, but when doubled or before an [s] it retained its spirantal sound and merged with the development of the primitive aspirate ✶[kʰ], which was spirantialized to [x] in Classical Adûnaic (SD/418-9). Also, [x] became [ɣ] before voiced stops [b], [d] and [g], thereafter developing as [ɣ]: disappearing while lengthening the following vowel (SD/422).

Note that Tolkien represented this sound differently in Primitive and Classical Adûnaic: “h” in primitive words and “kh” in classical words.