प्लुतप्रगृह्या अचि नित्यम्

The प्लुत (8.2.82 etc.) and प्रगृह्य (1.1.11 etc.) vowels remain unaltered when followed by a vowel (so far as the operation of that vowel is concerned).
Thus देवदत्ता ३ अत्र न्वसि, यज्ञदत्ता ३ इदम् आनय ॥ These are examples of prolated vowels. The prolation of vowels is taught in the tripadi or the last three chapters of Ashtadhyayi; and the tripadi are considered asiddha for the purposes of previous sutras 8.2.1. This is not the case here, otherwise the mention of pluta would be redundant. Of the Excepted or Pragrihya vowels the following are the examples : अग्नी इति, वायू इति, खट्वे इति, माले इति ॥ Though the anuvritti of अचि was current in this sutra from 6.1.77: its repetition here is for the sake of ordaining प्रकृतिभाव ॥ That is, the pluta and the pragrihya retain their original forms when such a vowel follows which would have caused a substitution. Thus जानु उ अस्य, रुजति here in combining जानु + उ into जानू, the अ is no cause of lengthening, therefore, the lengthening will take place. Now since उ is a pragrihya, it follows that जानू which ends in उ is also a pragrihya, and therefore it should not be changed before अ of अस्य ॥ Hence we have the form जानू + अस्य = जानू अस्य ॥ There may also be the form जान्वस्य, not by इकोयणचि, but by मय उञो वो वा 8.3.33. In fact, the repetition of अचि in this sutra is necessary, in order to enable us to give this peculiar meaning to the sutra. Otherwise, had the sutra been simply प्लुतप्रगृह्याः, it would have been translated ordinarily thus: "The Pluta and Pragrihya retain their form, when ever a vowel follows". Therefore in जानु उ अस्य, since उ is followed by a vowel, it will retain its form, and will not coalesce with the उ of जानु ॥ But this is not intended. Hence the repetition of the word अचि, and the peculiar explanation given above, namely, Pluta and Pragrihya vowels retain their form before a vowel which would otherwise have caused a substitution, but it would not prevent the operation of any other rule. Therefore in जानु उ अस्य रुजति, the उ + उ will become lengthened, because अ is not the cause of lengthening; it causes the change of उ to व्, which of course is prevented. The word नित्य of the last sutra is understood here also. These pluta and pragrihya always retain their form and are not influenced by the rule of shortening given in 6.1.127.