The Yajurveda Samhita consists of prose mantras. It is a compilation of ritual offering formulas that were said by a priest while an individual performed ritual actions such as those before the yajna fire. The earliest and most ancient layer of Yajurveda samhita includes about 1,875 verses, that are distinct yet borrow and build upon the foundation of verses in Rigveda. Unlike the Samaveda which is almost entirely based on Rigveda mantras and structured as songs, the Yajurveda samhitas are in prose and linguistically, they are different from earlier Vedic texts. The Yajur Veda has been the primary source of information about sacrifices during Vedic times and associated rituals. There are two major groups of texts in this Veda: the "Black" (Krishna) and the "White" (Shukla). The term "black" implies "the un-arranged, motley collection" of verses in Yajurveda, in contrast to the "white" (well arranged) Yajurveda. The White Yajurveda separates the Samhita from its Brahmana (the Shatapatha Brahmana), the Black Yajurveda intersperses the Samhita with Brahmana commentary. Of the Black Yajurveda, texts from four major schools have survived (Maitrayani, Katha, Kapisthala- Katha, Taittiriya), while of the White Yajurveda, two (Kanva and Madhyandina). The youngest layer of Yajurveda text is not related to rituals nor sacrifice, it includes the largest collection of primary Upanishads, influential to various schools of Hindu philosophy.