The musical scale contains 7 + 5 notes. (7-shuddha & 5 komal and tivra)( flat and sharp).
From these 12 notes selected notes (minimum 5) are used to build up a melodic structure and are called a Raga. It is believed that there are about 300 Ragas. However, only around 50 are heard in performances. The lesser known and difficult to perform Ragas are called “Achhop’ and ‘Anavat’ or ‘Aprachalit’ respectively.
Every Raga has melodic centers, two of which include – one of principal importance and the other a little less important and are called Vadi’ and ‘Samvadi respectively. Notes which support Vadi and Samvadi notes are called Anuvadi notes. Notes that are excluded from the Raga structure are called Vivadi notes.
Ragas with melodic movements centered in the first half of the scale (Sa to Pa) is called “Purvang Ragas and the second half of the scale (Ma to upper octave Sa) are called ‘Uttarang Ragas’.
According to tradition, every Raga has a definite time period for performance during a time cycle of 24 hours. A large section of the music community accepts this concept even at present. Some believe that the concept is scientifically based on an intimate relationship between sound and light.