The Taittiriya Upanishad belongs to the Krishna Yajurveda and forms the seventh, eighth and ninth chapters of the Taittiriya Aranyaka. Its three chapters are called vallis: the Shiksha Valli on learning and conduct, the Brahmananda Valli on Brahman and its bliss, and the Bhrigu Valli, named for Rishi Bhrigu, son of Varuna.
The Shiksha Valli teaches how the Veda is to be sounded — letters, accent, length, force, evenness and joining — and gives the teacher's charge to a student whose study is done: speak the truth, walk in dharma; let mother, father, teacher and guest be as gods to you; give with faith, in plenty, with modesty and with understanding. The Brahmananda Valli calls Brahman truth, knowledge and the infinite, and describes the person as five sheaths, one within another, made of food, breath, mind, understanding and bliss. It measures joy in hundredfolds, rising from the joy of a young, strong and learned man to whom the whole earth and its wealth belong; the bliss of Brahman itself is that from which words turn back with the mind. In the Bhrigu Valli, Rishi Bhrigu asks Varuna to teach him Brahman and is sent each time to tapas, until he knows it as food, then breath, mind and understanding, and at last as bliss.
Its opening prayer, sham no mitrah sham varunah — may Mitra be gracious to us, may Varuna be gracious — gives the Indian Navy its motto, Sham No Varunah, and atithi devo bhava names the Ministry of Tourism's campaign for welcoming visitors. The chant saha nau avatu — may it protect us both, teacher and student — opens its second chapter. It comes seventh in the Muktika Upanishad's list of the ten principal Upanishads.