Structure of Raaga
The various formations of scales become the basis of the Raaga. That is to say, the infinite permutations and combinations obtained by the arrangement of the seven Svars plus all the Srutis, provide the basis for the formation of innumerable tunes or melodies which are called the Raagas and Raaginis of Indian music.
Features of Raaga
There are certain essential features which characterize every Raaga. A Raaga must have at least five notes, and it cannot omit Sa and must also contain some form of ma or pa as well. A Raaga uses a certain selection of tones: ones that are omitted are termed "forbidden" and cannot be used without destroying the Raaga. There are many Raagas have strong tonal centers, called Vadi and Samvadi. For Gaur Sarang, for example, these are G and D respectively. Typically, these two notes are a fourth or a fifth apart. The Vadi-Samvadi does not substitute for the importance of the tonal center Sa in a Raaga, and do not always function the same way for each rag. Certain moods are typically associated with each Raaga, and often a time of day or season of the year. Again, Gaur Sarang is an afternoon Raaga with the moods of peace and pathos. Prescribed melodic movements that often recur, like catch phrases, identify the Raaga. In Gaur Sarang one such phrase is G R m G. There can be precise use of timbres and tonal shading, heightened by the use of microtonal pitches that vary from one rag to another, lending particular character to the rag. Raga is a combination of sounds along with "Varnas". A "Raga" is particularly the different series of notes within the octave which forms the basis of all Indian Raagas. The source of Raaga is Thala. Similarly two notes of same denomination should not come one after another. Again in a Raaga notes 'Pa' and 'Ma' cannot be put together. A Raga should have Varnas that include "Aroha" and "Abaroha" and it must also have fixed "Vadi" and a "Samavadi" note.
Raaga and Time
The Raagas are essentially time bound, and divisions are made with the kinds and exactly when to be sung. Since ancient ages, there is a firm belief that a Raaga must be performed at a fixed time or season, because it is assumed that every Raaga is harmonized with the natural disposition of each person.
The Raagas are generally governed by time and seasons. According to the division of the day and night, the Raagas have also been divided into groups, due to the sensations they emote when performed. The Raagas for the diurnal period are called Dinegeya or Suryamsa Raagas, whereas for the nocturnal are called Ratrigeya or the Chandramsa Raagas. And the sub-divisions are named as Sandhiprakash Raagas - the Raagas sung during the twilight, Pratah-Sandhi Raagas - the Raagas sung during the transitional period of night and morning. It is also stated that a day is divided into eight Praharas, or the quarters or watches, each lasting for three hours. And the Raagas that are sung are divided according to the importance of the notes in every one of them. The octave is further divided into Uttaranga and Poorvanga, denoting the upper and lower cases of the musical octave.