Definition: a letter or a group of letters attached to a word which is not seen in actual use in the spoken language: cf अप्रयोगी इत्, Śāk. I.1.5, Hem.1.1.37. The इत् letters are applied to a word before it, or after it, and they have got each of them a purpose in grammar viz. causing or preventing certain grammatical operations in the formation of the complete word. Pāṇini has not given any definition of the word इत्, but he has mentioned when and where the vowels and consonants attached to words are to be understood as इत्; (cf. उपदेशेजनुनासिक इत्, हलन्त्यम् । etc. P. I.3.2 to 8) and stated that these letters are to be dropped in actual use, cf.P.I.3.9. It appears that grammarians before Pāṇini had also employed such इत् letters, as is clear from some passages in the Mahābhāṣya as also from their use in other systems of grammar as also in the Uṇādi list of affixes, for purposes similar to those found served in Pāṇini 's grammar. Almost all vowels and consonants are used as इत् for different purposes and the इत् letters are applied to roots in the Dhātupāṭha, nouns in the Gaṇapāṭha, as also to affixes, augments and substitutes prescribed in grammar. Only at a few places they are attached to give facility of pronunciation. Sometimes the इत् letters, especially vowels, which are said to be इत्, when uttered as nasalized by Pāṇini, are recognised only by convention; cf. प्रतिज्ञानुनासिक्याः पाणिनीयाः(S.K.on P.I.3.2).The word इत्, which literally means going away or disappearing, can be explained as a mute indicatory letter. In Pāṇini's grammar, the mute vowel अ applied to roots indicates the placing of the Ātmanepada affixes after them, if it be uttered as anudātta and of affixes of both the padas if uttered svarita; cf. P.I.3. 12, 72. The mute vowel आ signifies the prevention of इडागम before the past part, affixes; cf. P. VII. 2. 16. Similarly, the mute vowel इ signfies the augment न् after the last vowel of the root; cf.P.VII.1.58; ई signifies the prevention of the augment इ before the past part.affixes cfP.VII.2.14;उ signifies the inclusion of cognate letters; cf.P.I.1.69, and the optional addition of the augment इ before त्वा; cf. P.VII.2. 56; ऊ signifies the optional application of the augment इट्;cf.P.VII. 2.44; क signifies the prevention of ह्रस्व to the vowel of a root before the causal affix, cf.VII.4.2: लृ signifies the vikarana अङ् in the Aorist cf P.III.1.55; ए signifies the prevention of vrddhi in the Aorist,cf. P.VII.2.55; ओ signifies the substitution of न् for त् of the past part. cf. P VIII.2.45; क् signifies the Prevention of गुण and वृद्धि, cf.P, I. 1.5; ख् signifies the addition of the augment मुम्(म्)and the shortening of the preceding vowel: cf.P.VI.3 65-66: ग् signifies the prevention of गुण and वृद्धि, cf. P.I.1.5 घ् signifies कुत्व, cf. P.VII.3.62; ङ्, applied to affixes, signifies the prevention of गुण and वृद्धि, cf. P.I.1.5; it causes संप्रसारणादेश in the case of certain roots, cf. P. VI.1.16 and signifies आत्मनेपद if applied to roots; cf. P.I. 3.12, and their substitution for the last letter if applied to substitutes. cf. P I.1.53. च् signifies the acute accent of the last vowel;cf.P.VI.1. 159; ञ् signifies उभयपद i.e the placing of the affixes of both the podas after the root to which it has been affixed;cf.P.I.3.72, ट् in the case of an augment signifies its application to the word at the beginning: cf.P I.1.64, while applied to a nominal base or an affix shows the addition of the fem. affix ई (ङीप्) cf.P.IV.1. 15;ड् signifies the elision of the last syllable; cf. P.VI.4.142: ण् signifies वृद्धि, cf.P.VII.2.115;त् signifies स्वरित accent, cf. VI.1.181, as also that variety of the vowel (ह्रस्व, दीर्ध or प्लुत) to which it has been applied cf. P.I.1.70; न् signifies आद्युदात्त, cf. P.VI.1.193:प् signifies अनुदात्त accent cf. अनुदात्तौ सुप्पितौ P. III.1.4. as also उदात्त for the vowel before the affix marked with प् cf. P.VI.1.192: म् signifies in the case of an augment its addition after the final vowel.cf.P.I.1.47,while in the case of a root, the shortening of its vowel before the causal affix णि,cf. P.VI.4.92: र् signifies the acute accent for the penultimate vowel cf. P.VI.1.217，ल् signifies the acute accent for the vowel preceding the affix marked with ल्; cf.P.VI. 193; श् implies in the case of an affix its सार्वधातुकत्व cf. P. II1.4.113, while in the case of substitutes, their substitution for the whole स्थानिन् cf P.I.1.55; प् signifies the addition of the fem. affix ई (ङीप्) cf.P.IV-1.41;स् in the case of affixes signifies पदसंज्ञा to the base before them, cf P.I.4.16. Sometimes even without the actual addition of the mute letter, affixes are directed to be looked upon as possessed of that mute letter for the sake of a grammatical operation e.g. सार्वधातुकमपित् P.I.2.4; असंयेागाल्लिट कित् P.I.2.5: गोतो णित् P.VII.1.90 etc. (2) thc short vowel इ as a substitute: cf. शास इदङ्हलोः P.VI.4.34.