The alap is the opening section of a typical North Indian classical performance. It is unmetered, improvised (within the raga) and unaccompanied (except for the tanpura drone), and started at a slow tempo. In instrumental performance and dhrupad singing, this part receives heavy emphasis and can last for more than an hour; in the more popular modern vocal style of khyal, generally less so.
Instead of wholly free improvisation, many musicians perform alap schematically, for example by way of vistar, where the notes of the raga are introduced one at a time, so that phrases never travel further than one note above or below what has been covered before. In such cases, the first reach into a new octave can be a powerful event.