Ancient Quenya Phonetics P7: [r] became [s] after voiceless stops and aspirates
AQ. [r] became [s] after voiceless stops and aspirates; [{ptkpʰtʰkʰ}r] > [{ptkpʰtʰkʰ}s]
In Ancient Quenya any r following a voiceless stop or aspirate developed into s, a sound change with nearly identical descriptions in both the Outline of Phonetic Development [OP1] from the 1930s and the Outline of Phonology [OP2] from the 1950s: