In the chandas (Vedas), ṣ is substituted for the s of the particle su when preceded by a word having in it the change-effecting letter i etc.,
Thus अभी षु णः सखीनाम् (Rig. IV. 31. 3) ऊर्द्धं ऊ षु णः (Rig. I. 36. 13). The lengthening takes place by 6.3.134, and न changed to ण by 8.4.27.,