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Indian music is one of the major microtonal traditions of the world (along with the Middle Eastern family of traditions).

The unit of measurement in the Indian system is the shruti (sometimes spelled šruti, sruti or shruthi), which corresponds roughly to a quarter-tone. There are 22 shrutis per octave, 13 per fifth and 9 per fourth. A size of 4 shruti for the major whole tone follows from that. The step sizes of the heptatonic scales (shadja grama, madhyama grama) are given as sequences of 4, 3, and 2 shruti.

There is no single standardized non-ambiguous definition of the exact sizes of all shruti intervals.
An explanation about the shruti system and one traditional derivation of the 22 shrutis is available here.
Another example of a compilation of the shrutis, with explicit values of the intervals, can be found here.

The system has been approximated by 22edo, though the traditional tuning system is unequal, and split-shruti systems may approximated by the 22&34d (or 12&22 or even 10&12 generalized Diaschismic if simplicity or accuracy is no object) temperament.
The Wikipedia entry on shrutis gives a quite accurate approximation of the shruti system as a 22-note subset of 53edo . (See also a discussion on the Yahoo tuning list.)
Some derivations in the light of modern temperament theory: Magic22 and Shrutar22 as srutis

Other links

Article on indian tuning in Joe Monzo's tuning encyclopedia
Patrick Moutal's Indian Music Page
Hindustani Music Resources - downloads and links, compiled by Matt Rahaim
http://www.musicresearch.in - an indian music research portal
www.chandrakantha.com - Indian classical music- portal on indian music, with detailed descriptions of a number of north indian rags
http://22shruti.com