Sindarin Phonetic Development (Part 38)

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OS. [p], [t], [k] became spirants before [s]; [ps|ts|ks] > [ɸs|θθ|xs]

In both (Old) Sindarin and (Old) Noldorin, voiceless stops [p, t, k] became spirants before [s]. For [ps], [ks], the results were [ɸs], [xs]. For [ts], the development was somewhat different, with [ts] > [tθ] > [θθ] > [θ]. Tolkien described both these changes in his notes on Noldorin use of the Feanorian Alphabet from the 1930s:

Sindarin Phonetic Development (Part 31)

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OS. initial [s] unvoiced following consonants; [s{jwmnrl}-] > [{j̊w̥m̥n̥l̥r̥}-]

In the Primitive Eldarin of the 1930s to 1970s, many initial consonants could have an s-prefix (PE18/43, 93). For s-prefixed voiced consonants, the resulting combinations [sr-, sl-, sj-, sw-, sm-, sn-] produced corresponding voiceless consonants in (Old) Sindarin: [r̥-, l̥-, j̊-, w̥-, m̥-, n̥-].

Sindarin Phonetic Development (Part 29)

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OS. voiceless stops aspirated after consonants except [s]; [{ptkmnŋlr}{ptk}] > [{ptkmnŋlr}{ptk}ʰ]

In both Old Sindarin and Old Noldorin, voiceless stops [p, t, k] developed into spirants medially after nearly every other primitive consonant, in particular: all liquids [r, l], all nasals [m, n, ŋ], all aspirates [pʰ, tʰ, kʰ] and all other voiced and voiceless stops [b, d, g, p, t, k], that is essentially everything but [s]. In the last two cases, the both stops in the pair both became spirants, with various later vocalic effects (see below).