अष्टनो दीर्घात्

The असर्वनामस्थान case-endings after अष्टन् 'eight', have acute accent, when it gets the form अष्टा।
The word अष्टन् has two forms in the accusative plural and the other cases that follow it, namely अष्टा and अष्ट ॥ The affixes of accusative plural &c are udatta after the long form अष्टा and not after अष्ट ॥ Thus अष्टा॒भिः꣡ opposed to अष्ट꣡भिः, अष्टा॒भ्यः, con. अष्ट꣡भ्यः, अष्टा॒सु꣡ con. अष्ट꣡सु ॥ The word अष्ट꣡न् has acute on the last syllable, as it belongs to the class of घृतादि words (Phit I.21); and by 6.1.180 the accent would have been on the penultimate syllable. This rule debars it. The use of दीर्घात् indicates that the word अष्टन् has two forms, and the substitution of long अ taught in 7.2.84, thus becomes optional, because of this indication. Otherwise the employment of the word दीर्घात् would be useless, for by 7.2.84 which is couched in general terms, अष्टन् would always end in a long vowel. There is another use of the word दीर्घात्, namely, it makes the word अष्टान् with long आ 7.2.84 to get also the designation of षट् ॥ For if अष्टान् was not to be called a षट्, like अष्टन्, then there would be scope to the present sutra in the case of अष्टान् while it would be debarred in the case of अष्टन् without long आ, by the subsequent rule 6.1.180 which applies to षट् word, and hence the employment of the word दीर्घात् would become useless.