Ancient Quenya Phonetics P18: initial [s] plus continuant became voiceless continuant
AQ. initial [s] plus continuant became voiceless continuant; [sm-|sn-|sr-|sl-|sj-|sw-] > [m̥-|n̥-|r̥-|l̥-|j̊-|w̥-]
In Ancient Quenya, any initial [s] appearing before a voiced continuant vanished, unvoicing the following consonant in the process. This occurred before nasals m, n, liquids l, r, and the semi-vowels y, w. Tolkien described this process in some detail in both the first and second versions of his outlines of Quenya phonetic development (OP1, OP2) from the 1930s and 1950s respectively.