A thaat is a division comprising of specific ragas. This system was evolved by Pandit Bhatkhande, who was a keen musicologist. He divided ragas into 10 divisions, each comprising a specific list of ragas which are similar / have the same notes. If we go according to the time theory, then the names of the ragas starting from early morning to late night could be (1) Bhairav (2) Bhairavi (3) Asavari (4) Todi (5) Bilawal (6) Kafi (7) Khamaj (8) Purvi (9) Marwa (10) Kalyan. All the ragas have been tabulated under these headings denoting the 10 thaats. This could be treated as a general guideline and not something which is sacrosanct.
Pandit Bhatkhande also noticed that several ragas are known by different names and had different structures in different parts of the country. Hence, he convened a meeting (in 1928) of all known musicians and music related personalities, to discuss how to standardize the raga system. Different opinions with regard to the names of the ragas as well as their structures were expressed during the meeting. After obtaining concurrence of all the entities, Bhatkhandeji could succeed in having a common nomenclature for different ragas.
With regard to their structures, he could get an agreement from the musicians that the different versions of a raga, could be described as prakars of that raga. Example, Desi Todi – with shuddha Dha, with komal Dha and both the Dha’s – three prakars of Desi Todi. This was a very valuable work and the resultant classification has been accepted by the music fraternity.