उपदेशेऽजनुनासिक इत्

The nasalised vowels are इत् in उपदेश or original enunciation.
Anunasika or nasalized vowels are इत् that is indicatory, when occurring in technical words. Thus in क्विन्, विद्, विच् the anunasika इ is इत्. The original sutra contains the word 'upadesa' which we have translated as technical term. 'Upadesa' literally means 'instruction' or the first mentioning of a thing either in a sutrapatha, or ganapatha, or dhatu patha, &c.

From this sutra up to sutra 1.3.9, there is a description of servile or indicatory letters called इत्. "In Panini's Grammar there is no visible sign of the nasality of a vowel -- hence we can know a vowel to be nasalized only from Panini's explicitly asserting that it is so, or from our finding that he treats it in such a way that we must conclude he regarded it as nasal." An upadesa is defined as signifying an original enunciation, that is to say, an affix (pratyaya), or an augment (agama), or a verbal root (dhatu), or in short, any form of expression which occurs only in technical treatises of grammar, and which is not a word ready for use, but one of the supposed original elements of a word. Thus in the root एध 'to increase,' the final अ is indicatory, the real root is एध्. So also the final अ of स्पर्द्ध is इत् ॥

If the word is not an upadesa, then the nasal vowel is not इत् ॥ As अभ्र आं अपः ॥ A word may be an upadesa, and nasal, but if it is not a vowel, it need not be इत् as the affix मनिन् in sutra 3.2.74. (आतो मनिन् क्वनिब् वनिपश्च) It is only the nasal vowel of an upadesa that becomes इत् and not all the vowels. The word इत् occurs in sutras 5.2.16. (आदितश्च &c.).