Bengali researcher Bimalakanto Raychoudhuri in his "Bharatiya Sangeetkosh" suggests classification both by length ("aochar" being the shortest, followed by "bandhan", "kayed" and "vistar") and by performance style (according to the four ancient "vanis" or singing styles – "Gohar", "Nauhar", "Dagar" and "Khandar"), and proceeds to list thirteen stages: