In the Old Noldorin of the 1930s, it seems a final [e] following an [s] or [st] shifted to [a], unless it followed a double [ss], in which case the [e] was preserved. The clearest examples of this rule are ᴹ✶khyelesē > khelesa > ON. kheleha (Ety/KHYEL(ES)) and ᴹ✶peltakse > ON. pelthaksa (Ety/PEL). In the case of [st], note that ON. phasta has the cognate ᴹQ. fasse (Ety/PHAS), which implies a primitive form ending in [e] and thus a similar change of [e] to [a] in Old Noldorin after [st].

For counter-examples involving [ss], consider ON. (s)pharasse in which the final [e] is preserved, and similarly for its (rejected) precursor pharasse (Ety/SPAR, EtyAC/PHAR²).

Since later in Noldorin, short final vowels vanished, in many cases this phonetic change in Old Noldorin is not noticeable; whether the final vowel was -e or -a before it vanished has little effect. In a few cases, though, this change to -a in Old Noldorin can have further ramifications, in particular via a-affection, whereby short [i], [u] became [e], [o] preceding final [a].

There are only two attested examples of primitive forms ending in -se where the a-affection may also apply. The first example is ᴹ✶tupsē > N. taus (Ety/TUP). Here, the ON. shift to final -a means that the medial vowel [u] became [o], which means that after the voiceless stop was spirantalized and then vocalized before the [s], it produces the same final result as [o] in this position: the diphthong [au]. Compare this example to ᴹ✶oktā > N. auth (Ety/KOT).

Conversely, consider ᴹ✶tyulussē > N. tulus (Ety/TYUL). In this case, since the [u] remains, it implies that a-affection did not occur, indicating that the final -e was preserved after the double [ss], as noted above.

There are a few examples that indicate a similar change in Sindarin. One example is the primitive forms rinsa that is the basis for S. riss, although S. riss < ✶rinsa does not show the expected a-affection (PE17/87). There is also primitive ✶kirtē in the Quendi and Eldar essay of 1959-60 that produced S. certh (WJ/396); the a-affection in the final form indicates a change of final -ē > -ā. This last development indicates this change was a more general phenomenon beyond just changes after primitive s, but without more examples it is hard to figure out the details.

See OS. final [e] became [a] after single [s] and [st] for discussion.

@@@ also in Sindarin?