Grammatical studies must have been cultivated to a considerable extent before Yaska's time, for he distinguishes a Northern and an Eastern school, besides mentioning nearly twenty predecessors, among whom Çakatayana, Gargya, and Çakalya are the most important. By the time of Yaska grammarians had learned to distinguish clearly between the stem and the formative elements of words; recognising the personal terminations and the tense affixes of the verb on the one hand, and primary (krit) or secondary (taddhita) nominal suffixes on the other. Yaska has an interesting discussion on the theory of Çakatayana, which he himself follows, that nouns are derived from verbs. Gargya and some other grammarians, he shows, admit this theory in a general way, but deny that it is applicable to all nouns. He criticises their objections, and finally dismisses them as untenable. On Çakatayana's theory of the verbal origin of nouns the whole system of Panini is founded. The sutra of that grammarian contains hundreds of rules dealing with Vedic forms; but these are of the nature of exceptions to the main body of his rules, which are meant to describe the Sanskrit language. His work almost entirely dominates the subsequent literature. Though belonging to the middle of the Sutra period, it must be regarded as the definite starting-point of the post-Vedic age. Coming to be regarded as an infallible authority, Panini superseded all his predecessors, whose works have consequently perished. Yaska alone survives, and that only because he was not directly a grammarian; for his work represents, and alone represents, the Vedanga "etymology." Yaska's Nirukta is in reality a Vedic commentary, and is older by some centuries than any other exegetical work preserved in Sanskrit. Its bases are the Nighantus, collections of rare or obscure Vedic words, arranged for the use of teachers. Yaska had before him five such collections. The first three contain groups of synonyms, the fourth specially difficult words, and the fifth a classification of the Vedic gods. These Yaska explained for the most part in the twelve books of his commentary (to which two others were added later). In so doing he adduces as examples a large number of verses, chiefly from the Rigveda, which he interprets with many etymological remarks.