According to Vishnu Narayan Bhatkhande (1860-1936), one of the most influential musicologists in the field of North Indian classical music in the twentieth century, each one of the several traditional ragas is based on, or is a variation of, ten basic thaats, or musical scales or frameworks. The ten thaats are Bilawal, Kalyan, Khamaj, Bhairav, Poorvi, Marwa, Kafi, Asavari, Bhairavi and Todi; if one were to pick a raga at random, it should be possible to find that it is based on one or the other of these thaats. For instance, the ragas Shree and Puriya Dhanashri are based on the Poorvi thaat, Malkauns on the Bhairavi, and Darbari Kanada on the Asavari thaat. It is important to point out that Bhatkande's thaat-raga theory is hardly infallible, but it is nevertheless an important classificatory device with which to order, and make sense of, a bewildering array of ragas; and it is also a useful tool in the dissemination of the music to students.
It is worth noting that almost all the thaats mentioned above are also ragas; and yet a thaat is a very different musical entity from a raga, and in this difference may lie, crucially, a definition of what a raga is or is not. A thaat is a musical scale, conceived of as a Western musical scale might be, with the seven notes presented in their order of ascent (arohan). For instance, Asavari is presented, and notated, as Sa Re Ga (flat or komal) Ma Pa Dha (flat) Ni (flat) in ascent, or arohan. This is, however, only the skeletal musical structure of the raga Asavari, an abstraction that is to be found nowhere but on the printed page or inside a textbook; the raga Asavari, in reality, and in exposition, is a very different thing. It goes straight from Re to Ma, and comes down to touch Ga, as it ascends; having touched Ni later, it returns to Pa, and, touching the upper Sa, returns to Dha and Pa again and again. Arohan and avarohan are, thus, inextricably and inseparably intermingled in the structure of this raga. The raga, then, is not a musical scale in the Western sense; it is a characteristic arrangement or progression of notes whose full potential and complexity can be realised only in exposition, and not upon the printed page. A condensed version of this characteristic arrangement of notes, peculiar to each raga, may be called the pakad, by which a listener hears the phrase Sa Re Ga Ma Pa Ga, none of these notes being flat or sharp. Repeated in a recital, they will know that they are listening to the raga Gaud Sarang.
Two ragas may have identical notes and yet be very different ragas; for example, two ragas mentioned earlier, Shree and Puriya Dhanashri, have exactly the same notes, but are unmistakably different in structure and temperament. The first can be identified by its continual exploration of the relationship of the note Re to the note Paa; while the repetition of the phrase Ma Re Ga Re Ma Ga, a phrase that would be inadmissible in the first raga, is an enduring feature of the latter. Certain arrangements of notes, then, are opposite to particular ragas and taboo to all others. A simple and abstract knowledge, thus of the notes of a raga or the thaat on which it is based, is hardly enough to ensure a true familiarity or engagement with the raga, although it may serve as a convenient starting point. Thaat familiarity can only come from a constant exposure to, and critical engagement, with raga's exposition.
For further information pertinent to the definition of a raga, please refer to the glossary.