I said that every doctrine or system has a sutra (text consisting of aphoristic statements), a bhasya (commentary) and a vartika (elucidation of the commentary). The systems founded by Sankara, Ramanuja, Madhva, Srikantha (acarya of Saiva-Sidhanta) belong to Vedanta. All these acaryas cite the authority of the Vedas in support of their respective doctrines and they have chosen the same ten Upanisads to comment upon according to their different philosophical perceptions. The Upanisads are not in the form of sutras; yet for the Vedantic system they must be regarded as having the same "place" (or force) as the sutras.
How is a sutra to be understood? It must state truths in an extremely terse form. What is expressed in the least possible number of words to convey an idea or truth is a sutra, an aphorism. According to this definition the Upanisads cannot be said to be sutras. However, there does exist a basic text for all Vedantic schools in the form of sutras. This is the Brahmasutra.
In the Brahmasutra, on which there are commentaries according to the various philosophical schools, Vyasa presents in an extremely terse form the substance of the ten (principal) Upanisads. Since he dwelt under the badari tree (jujube) he came to be called "Badarayana" and his work became well-known as "Badarayana-sutra". Who or what is man (the individual self)? What is the nature of the world (jagat) in which he lives? And what is the truth underlying all this? The Brahmasutra, which is a basic text of all Vedantic schools, seeks to answer these fundamental questions. Vyasa does not project his personal views in his work. All he does is to make a penetrating study of the science of Vedanta that is already constituted by the Upanisads. Since it is an inquiry into the Upanisads which form the latter part of the Vedas, the Brahmasutra is called "Uttaramimamsa"
There are 555 sutras in the Brahmasutra which is divided into four chapters, each consisting of four padas (or "feet"). Altogether there are 192 sections or "adhikaranas" in it. The Brahmasutra is also called "Bhiksu-sutra" since it deals with sannyasa, the final goal of the seeker. And, because it is all about the Self in the body, it has another name, "Sariraka".
"Sutra" literally means a rope or string. The word occurs in the term "mangala-sutra", the thread worn by the bride at her wedding. Keeping the meaning of thread or string in mind, our Acarya has made a pun on the word in his commentary: "Vedanta-vakya-kusuma-grathanarthatvat sutranam". If the flowers that are Upanisads in the tree called the Vedas are strewn all over the earth, how can we gather them to make a garland? Our Acarya remarks that in the Brahmasutra the flowers are the Upanisads are strung together to form a garland.
All Hindu philosophical systems are based on the Brahmasutra, but the Brahmasutra itself is based on the Upanisads. That is why it has become customary to describe all Vedic schools of thought as "Upanisadic systems". When Westerners keep extolling our philosophy, chanting, "Vedanta! Vedanta!", they have in mind the Upanisads. If a person turns against the petty pleasures of this world and makes a remark suggestive of jnana, people tell him, "Arre, are you mouthing Vedanta? "
If the Vedas were personified as Purusa, the Upanisads would be his head or crown. That is why these texts are called "Sruti-siras".
For a general background, please see here