Of kalpa we have already treated at length above. No work representing astronomy has survived from the Vedic period; for the Vedic calendar, called jyotisha, the two recensions of which profess to belong to the Rigveda and Yajurveda respectively, dates from far on in the post-Vedic age. The Taittiriya Aranyaka (vii. 1) already mentions çiksha, or phonetics, a subject which even then appears to have dealt with letters, accents, quantity, pronunciation, and euphonic rules. Several works bearing the title of çiksha have been preserved, but they are only late supplements of Vedic literature. They are short manuals containing directions for Vedic recitation and correct pronunciation. The earliest surviving results of phonetic studies are of course the Samhita texts of the various Vedas, which were edited in accordance with euphonic rules. A further advance was made by the constitution of the pada-patha, or word-text of the Vedas, which, by resolving the euphonic combinations and giving each word (even the parts of compounds) separately, in its original form unmodified by phonetic rules, furnished a basis for all subsequent studies. Yaska, Panini, and other grammarians do not always accept the analyses of the Padapathas when they think they understand a Vedic form better. Patanjali even directly contests their authoritativeness. The treatises really representative of Vedic phonetics are the Pratiçakhyas, which are directly connected with the Samhita and Padapatha. It is their object to determine the relation of these to each other. In so doing they furnish a systematic account of Vedic euphonic combination, besides adding phonetic discussions to secure the correct recitation of the sacred texts. They are generally regarded as anterior to Panini, who shows unmistakable points of contact with them. It is perhaps more correct to suppose that Panini used the present Pratiçakhyas in an older form, as, whenever he touches on Vedic sandhi, he is always less complete in his statements than they are, while the Pratiçakhyas, especially that of the Atharva-veda, are dependent on the terminology of the grammarians. Four of these treatises have been preserved and published. One belongs to the Rigveda, another to the Atharva-, and two to the Yajur-veda, being attached to the Vajasaneyi and the Taittiriya Samhita respectively. They are so called because intended for the use of each respective branch (çakha) of the Vedas.