Q. [sp], [sk] became [ps], [sk]; [sp|sk|skw] > [ps|ks|skw]
In Quenya the combinations sp, sk were frequently transposed to ps, ks: universally in the case of medial ps, sporadically in other cases. This sound change happened both initially and medially, and the initial transpositions introduced the new initial clusters ps-, ks- to Quenya. Although established in Early Qenya, the frequency and nature of these sound changes evolved over Tolkien’s life.