In Bhatkhande's system, the basic mode of reference is that which is equivalent to the Western Ionian mode or major scale (called "Bilawal" thaat in Hindustani music, "Dheerasankarabharanam" in Carnatic). The flattening or sharpening of pitches always occurs with reference to the interval pattern in Bilawal thaat. Each thaat contains a different combination of altered ("vikrt") and natural ("shuddha") notes with respect to the Bilawal thaat. In any seven-tone scale (starting with S), R, G, D, and N can be natural ("shuddha", lit. "pure") or flat ("komal", lit. "soft") but never sharp, whereas the M can be natural or sharp (, lit. "fast") but never flat, making twelve notes as in the Western chromatic scale. The sharp or flat tones are called ' (', lit. "altered"). Selecting seven tones in ascending order, where S and P are always natural whereas five other tones (R, G, M, D, N) can assume only one of its two possible forms, results in 32 possible modes which are known as "thaats". Out of these thirty-two possibilities, Bhatkhande chose to highlight only ten thaats prominent in his days.