Raga (राग) (singular rag (राग in Hindi), raga (Anglicised Sanskrit) or ragam (ராகம் in Tamil), plural ragas) are the very detailed melodic modes used in Indian classical music, not to be confused with scales.
A raga is basically a set of rules for how to build a melody. It specifies a scale, as well as rules for movements up (aarohanam) and down (avarohanam) the scale, which notes should figure more and which notes should be used more sparingly, which notes take which ornamentation, which notes must be bent, which notes may be bent, phrases to be used, phrases to be avoided, and so on. The result is a framework that can be used to compose or improvise melodies in, so that melodies in a certain raga will always be recognisable yet allowing endless variation.
The underlying scale is a five, six or seven tone-scale, made up of swaras. This provides one method of classifying ragas. Ragas that have five swaras are called audava ragas; those with six, shaadava; and with seven, sampoorna (Sanskrit for 'complete'). Those ragas that do not follow the strict ascending or descending order of swaras are called vakra ('crooked') ragas. (To see the order of notes, check the article on swara.)
In the seven tone-scale the second, third, fourth, sixth, and seventh notes can be sharp or flat, making up the twelve notes in the Western scale. However, ragas can specify microtonal changes to this scale: a flatter second, a sharper seventh, and so forth. Furthermore, such variations can occur between styles, performers or simply follow the mood of the performer. There is no absolute pitch; instead, each performance simply picks a ground note, and the other scale degrees follow relative to the ground note.
Every time of the day, morning, afternoon, evening and night, has its specific ragas. This distinction is strictly followed in Hindustani music (practised in North India), but is by and large neglected in Carnatic music (practised in South India).
Indian music can roughly be divided into northern and southern, and so can the ragas: north India has one set, and south India has another. There is some overlap, but more "false friendship" (where raga names overlap, but raga form does not). In north India, the ragas have recently been categorised into ten thaats or parent scales (by Vishnu Narayan Bhatkhande, 1860-1936); south India uses an older, more systematic classification scheme called the melakarta classification, sporting 72 parent (melakarta) ragas. Overall there is a greater identification of raga with scale in the south than in the north, where such an identification is impossible.
As ragas were never codified but transmitted orally from teacher to student, some ragas can vary greatly across regions, traditions and styles.
Indian classical music is always set in raga, but all raga music is not necessarily classical. Many popular Indian film songs are based on ragas.