The Taittirîyaka-upanishad seems to have had its original place in the Taittirîya-Âranyaka. This Âranyaka consists, as Rajendralal Mitra has shown in the Introduction to his edition of the work in the Bibliotheca Indica, of three portions. Out of its ten Prapâthakas, the first six form the Âranyaka proper, or the Karma-kânda, as Sâyana writes. Then follow Prapâthakas VII, VIII, and IX, forming the Taittirîyaka-upanishad; and lastly, the tenth Prapâthaka, the Yâgñikî or Mahânârâyana-upanishad, which is called a Khila, and was therefore considered by the Brâhmans themselves as a later and supplementary work.
Sankara, in his commentary on the Taittirîyaka-upanishad, divides his work into three Adhyâyas, and calls the first Sikshâ-vallî, the second the Brahmânanda-vallî, while he gives no special name to the Upanishad explained in the third Adhyâya. This, however, may be due to a mere accident, for whenever the division of the Taittirîyaka-upanishad into Vallîs is mentioned, we always-have three, the Sikshâ-vallî, the Brahmânanda-vallî, and the Bhrigu-vallî.
Properly, however, it is only the second Anuvâka of the seventh Prapâthaka which deserves and receives in the text itself the name of Sikshâdhyâya, while the rest of the first Vallî ought to go by the name of Samhitâ-upanishad, or Samhitî-upanishad.
Sâyana, in his commentary on the Taittirîya-âranyaka, explains the seventh chapter, the Sikshâdhyâya (twelve anuvâkas), as Sâmhitî-upanishad. His commentary, however, is called Sikshâ-bhâshya. The same Sâyana treats the eighth and ninth Prapâthakas as the Vâruny-upanishad.
The Ânanda-vallî and Bhrigu-vallî are quoted among the Upanishads of the Atharvana.
At the end of each Vallî there is an index of the Anuvâkas which it contains. That at the end of the first Vallî is intelligible. It gives the Pratîkas, i.e. the initial words, of each Anuvâka, and states their number as twelve. At the end of the first Anuvâka, we have the final words “satyam vadishyâmi,” and pañka ka, i.e. five short paragraphs at the end. At the end of the second Anuvâka, where we expect the final words, we have the initial, i.e. sîkshâm, and then pañka, i.e. five sections in the Anuvâka. At the end of the third Anuvâka, we have the final words, but no number of sections. At the end of the fourth Anuvlka, we have the final words of the three sections, followed by one paragraph; at the end of the fifth Anuvâka, three final words, and two paragraphs, though the first paragraph belongs clearly to the third section. In the sixth Anuvâka, we have the final words of the two Anuvâkas, and one paragraph. In the seventh Anuvâka, there is the final word sarvam, and one paragraph added. In the eighth Anuvâka, we have the initial word, and the number of sections, viz. ten. In the ninth Anuvâka, there are the final words of one section, and six paragraphs. In the tenth Anuvâka, there is the initial word, and the number of paragraphs, viz. six. In the eleventh Anuvâka, we have the final words of four sections, and seven paragraphs, the first again forming an integral portion of the last section. The twelfth Anuvâka has one section, and five paragraphs. If five, then the sânti would here have to be included, while, from what is said afterwards, it is clear that as the first word of the Vallî is sam nah, so the last is vaktâram.
In the second Vallî the index to each Anuvâka is given at the end of the Vallî.
In the third Vallî the Anukramanî stands at the end.
 Sankara (ed. Roer, p. 141) himself speaks of two Vallîs, teaching the paramâtmagñâna (the Sikshâ-vallî has nothing to do with this), and Anquetil has Anandbli = Ânanda-vallî, and Bharkbli = Bhrigu-vallî.
 The third Vallî ends with Bhrigur ity upanishat.
 See Taittirîyaka-upani shad, ed. Roer, p. 12.
 See M. M., Alpbabetisches Verzeichniss der Upanishads, p. 144.
 The Anukramaî of theÂtreyî school (see Weber, Indische Studien, II, p. 208) of the Taittirîyaka gives likewise the name of Vârunî to the eighth and ninth Prapâthaka, while it calls the seventh Prapâthaka the Sâmhitî, and the tenth Prapâthaka the Yâgñki-upanishad. That Anukramanî presupposes, however, a different text, as may be seen both from the number of Anuvâkas, and from the position assigned to the Yâgñki as between the Sâmhitî and Vârunî Upanishads.
 See M. M., Alphabetisches Verzeichniss der Upanishads.