A preposition ending in i or u lengthens its final before the verb dā when the latter is changed to ta,
Thus नीत्तम्, वीत्तम्, परीत्तम् ॥ See 7.4.47 for the change of दा to त ॥ But प्रत्तम्, अवत्तम्, where the prepositions do not end in इ or उ ॥ Why do we say 'दा is changed to त'? Observe वितीर्णम् नितीर्णम् ॥ Why do we say ति 'a substitute of दा, beginning with a त'? Observe सुदत्तम्, here दद् is the substitute of दा 7.4.46.,