अच उपसर्गात् तः

The examples under the present sutra are प्रत्तं, अवत्तं, नीत्तं, परीत्तं ॥ For the lengthening see 6.3.124. Why do we say 'ending in a vowel'? Observe निर्दत्तम् दुर्दत्तम् ॥ Why 'after an Upasarga only'? Observe दधि दत्तम्, मधु दत्तम् ॥ Why 'the दा called Ghu'? Observe अवदात्तं from दैप् ॥

Objection: The word उपसर्गात् in the sutra is in the Ablative case, and by 1.1.67, the substitute त् should replace only the first letter of दा, how does it replace the whole ?

Answer: The word अचः is to be repeated in the sutra, one अचः being in the Ablative case and qualifying उपसर्गात्, and the other अचः being in the Genitive case, showing the sthanin to be आ, as \after an Upasarga ending in vowel, त् is substituted for the vowel of दा\. Or the word अस्य may be read into this sutra from 7.4.32. Or उपसर्गात्त्तः consists of three त्'s, the substitute being त्त, and being a substitute of more than one letter, it replaces the whole of दा 1.1.55. In the following sutra अपोभि, the अचः in the Ablative case is understood, and therefore त् replaces only प् ॥

Vart:- After a Preposition ending in a vowel, त is substituted for दा (दो), when इ would have been substituted otherwise for its final by 7.4.40. As अवदत्तम्, प्रत्तम् जुहोति and नीत्तं, वीत्तं ॥,