Page "Upanishads" Paragraph 1

More than 200 are known, of which the first dozen or so, the oldest and most important, are variously referred to as the principal, main ( mukhya ) or old Upanishads.

With the Bhagavad Gita and the Brahmasutra ( known collectively as the Prasthanatrayi ), the mukhya Upanishads provide a foundation for several later schools of Indian philosophy ( vedanta ), among them, two influential monistic schools of Hinduism.