Tradition asserts that there are more than 200 Upanishads. Among them ten are the oldest and prominent, on which Adi Shankaracharya had written his commentaries. They are Isha, Kena, Katha, Prashna, Mundaka, Mandukya, Taittiriya, Aitareya, Chandogya and Brihadaranyaka. Some also add three other ancient Upanishads, the Shvetashvatara, Kaushitaki and Maitri, to this list of principal Upanishads. The Upanishads explain the spiritual truths through dialogues anecdotes, allegories and stories.