The first book is an introduction, dealing with the principles of grammar and exegesis. The second and third elucidate certain points in the synonymous nighantus; Books IV.-VI. comment on the fourth section, and VII.-XII. on the fifth. The Nirukta, besides being very important from the point of view of exegesis and grammar, is highly interesting as the earliest specimen of Sanskrit prose of the classical type, considerably earlier than Panini himself. Yaska already uses essentially the same grammatical terminology as Panini, employing, for instance, the same words for root (dhatu), primary, and secondary suffixes. But he must have lived a long time before Panini; for a considerable number of important grammarians' names are mentioned between them. Yaska must, therefore, go back to the fifth century, and undoubtedly belongs to the beginning of the Sutra period. One point of very great importance proved by the Nirukta is that the Rigveda had a very fixed form in Yaska's time, and was essentially identical with our text. His deviations are very insignificant. Thus in one passage (X. 29. I) he reads vayó as one word, against va yó as two words in Çakalya's Pada text. Yaska's paraphrases show that he also occasionally differed from the Samhita text, though the quotations themselves from the Rigveda have been corrected so as to agree absolutely with the traditional text. But these slight variations are probably due to mistakes in the Nirukta rather than to varieties of reading in the Rigveda. There are a few insignificant deviations of this kind even in Sayana, but they are always manifestly oversights on the part of the commentator. To the Sutras is attached a very extensive literature of Pariçishtas or "supplements," which seem to have existed in all the Vedic schools. They contain details on matters only touched upon in the Sutras, or supplementary information about subjects not dealt with at all by them. Thus, there is the Açvalayana Grihya-pariçishta, in four chapters, connected with the Rigveda. The Gobhila samgraha-pariçishta is a compendium of Grihya practices in general, with a special leaning towards magical rites, which came to be attached to the Samaveda. Closely related to, and probably later than this work, is the Karma-pradipa ("lamp of rites"), also variously called sama-grihya- or chhandogyagrihya-pariçishta, chhandoga-pariçishta, Gobhila-smriti, attributed to the Katyayana of the White Yajurveda or to Gobhila. It deals with the same subjects, though independently, as the Grihya samgraha, with which it occasionally agrees in whole çlokas.