This Ilkorin phonetic rule is similar to sound changes that occurred in Old Noldorin. The likely development was that the stops first became aspirates [pʰ], [tʰ] and [kʰ], after which the initial [s] was lost and then the aspirates became voiceless spirants [f-], [θ-] and [x-] (> [h-]), as indicated on the Comparative Tables (PE19/20). In The Etymologies, the only attested examples of this rule involve [st-] > [θ-] (see below).

This change did not occur where the initial [s] became syllabic: compare ᴹ√SPÁLAS > Ilk. espalass (Ety/PHAL).