The affix ṭhak comes in the sense of 'occurring therein' or 'commentary thereon' after a word of two syllables and a word ending in short ṛ and the words 1. brāhmaṇa 2. ṛk 3. prathama 4. puraścaraṇa 5. nāma 6. ākhyāta and 7. nāmākhyāta being the names of explainable works.,
This debars अण् and ठञ् ॥ I. Dissyllabic:- ऐष्टिकः, पाशुकः ॥ 2. Ending in ऋ :- चातुर्होतृकः, पांचहोतृकः ॥ So also ब्राह्मणिकः, आर्चिकः, प्राथमिकः, आध्वरिकः, पौरश्चरणिकः, नामिकः, आख्यातिकः, नामाख्यातिकः ॥,