In dhrupad, the long opening alap is sung not to words but to a fixed set of neutral syllables — nom, tom, ta, na, re, ri — collectively called nom-tom. These syllables let the voice move freely through the raag without the constraints of bandish text.
As the dhrupad alap proceeds it gathers a pulse (much like the instrumental jor), and the nom-tom syllables drive this rhythmic momentum, giving dhrupad its grave, surging character before the composition proper begins over chautal.
Nom-tom alap is one of the features that most distinguishes austere dhrupad from the later, text-rich khayal.