often written unmutated after a prefix and after yn, there is evidence that it was in fact mutated, § 107 iii, v.
iv. The nasal mutation of an explosive does not mean its disappearance, but its conversion into a nasal by the loosening of its stop. In annoeth ‘unwise’ < Brit.-Lat. *an-doct- the d became a continuation of the n, so that nn represents an n which is continued during the time it took to pronounce the original nd. As the W. tenues are really aspirated, that is t ≡ t‑h, see § 84 Note 1, when the stop was loosened the aspirate remained; thus nt, properly nt‑h, became nnh. That Early Ml. W. nh as in synhuir § 48 iv is short for nnh, is proved (1) by such spellings as morcannhuc, brennhin l.l. 120, and (2) by the fact that when it lost its aspirate after the accent it appeared as nn, as synnwyr r.m. 13, w.m. 20, while breenhin in which nn had become n after the long vowel, is brenin (not *brennin), and an original single n + h always gives n, as in glánaf for glánhaf, superlative of glân ‘clean’. It is clear therefore that the mutation of nt is strictly n‑nh, not n‑h.
§ 107. i. While initial mediae are nasalized after several numerals, initial tenues are nasalized only after yn ‘in’ and fy ‘my’, and this mutation is not original after fy.
ii. Taken in conjunction with the following noun, yn ‘in’ ( < Brit. *en) has a secondary accent, but fy ‘my’ (< Brit. *men < Ar. *mene gen. sg. of the 1st pers. pron.) is wholly unaccented—the emphasis when required is thrown on an auxiliary pronoun : ‘my head’ is not *fy mhen, but fy mhen i. This difference between yn and fy is old, for Brit. *en has kept its ‑n, but *men (already a proclitic in Brit. § 113 ii) had lost its ‑n before the O.W. period. This is clearly seen is phrases where the following word began with a vowel or an immutable initial; thus yn: ynn lann l.l. 120, in alld b.b. 64, in llan do. 63, 64, yn amgant do. 66, in llurv do. 65, etc.; but fy: mi-hun m.c., vy argluit b.b. 51, wi-llav-e (≡ fy llaw i) do. 50, vy llen do. 59, 62, etc. Thus yn before a consonant is necessarily a closed syllable, closed by its ‑n, while fy is an open syllable, ending with its vowel. The O.W. ny l.l. 120 ‘in its’ is probably n̥ y, with syllabic n̥ or n̥n, a pronunciation still often heard.
iii. After yn in Early Ml. mss., b and d are generally mutated,
