Niraval also known as Neraval or Sahitya Vinyasa is considered to be one of the important features in the extempore improvisation aspect (Manodharma Sangita) of Carnatic music

Carnatic music

Carnatic music is a system of music commonly associated with the southern part of the Indian subcontinent, with its area roughly confined to four modern states of India: Andhra Pradesh, Karnataka, Kerala, and Tamil Nadu...

.

Niraval is essentially the extempore construction, elaboration and improvisation of swaras for a particular line in the kriti

Kriti

-Structure:Kritis typically contain three parts#Pallavi. This is the equivalent of a refrain in Western music.#Anupallavi. The second verse, which is sometimes optional....

, within the framework of a talam

Tala (music)

Tāla, Taal or Tal is the term used in Indian classical music for the rhythmic pattern of any composition and for the entire subject of rhythm, roughly corresponding to metre in Western music, though closer conceptual equivalents are to be found in other Asian classical systems such as the notion...

 which brings out the Raga

Raga

A raga is one of the melodic modes used in Indian classical music.It is a series of five or more musical notes upon which a melody is made...

 bhava effectively. It is usually just one line from the charanam

Charanam

Charanam in Carnatic music is usually the end section of a composition which is sung after the anupallavi....

 part of the kriti

Kriti

-Structure:Kritis typically contain three parts#Pallavi. This is the equivalent of a refrain in Western music.#Anupallavi. The second verse, which is sometimes optional....

 and has to sit within the framework of a tala and highlight every rasa and bhava singularly.

For example : 4-beat niravals for Shyama Sastri's "sarojadala netri himagiri putri" set to Adi talam (double) in the Raga Shankarabharanam

Dheerasankarabharanam

Dheerashankarābharanam , commonly known as Shankarābharanam, is a rāga in Carnatic music . It is the 29th Melakarta rāga in the 72 melakarta rāga system of Carnatic music....

 can be constructed for the line "sama gana vinodhini guna" (like this : gpmgrs | s,ndpmgrs) in the charanam

Charanam

Charanam in Carnatic music is usually the end section of a composition which is sung after the anupallavi....

 of the song.

Repetition of the line in different ways within the entire range of the Raga phrases brings out the musician's imagination.

Palghat K.V. Narayanaswamy was known and is remembered today as a master of niraval singing.

Ideally, the line taken up for neraval must have a pleasant meaning, and must be descriptive. It should be something that the singer would want to sing over and over again.
For example:
a line which translates "The killer of Ravana, the monster," would not be an apt choice for neraval as it does not have a pleasant meaning.

Examples of Krithis containing lines best suited for neraval:

Meenakshi Me mudam in Poorvikalyani at "Madhurapuri Nilaye Manivalaye",

O Rangasayee in Kambodhi at "Bhuloka Vaikuntam ithiyani",

Saroja Dhala Netri in Sankarabharanam at "Sama Gana Vinodhini gunadhama Shyamakrishnanuthe",

Nidhichaala Sukhama in Kalyani at "Mamathabandhanayutha Narasthuthi Sukhama?" or "Sumathi Thyagaraja Keerthana Sukhama?",

Ethavunnara in Kalyani at "Seethagowri Vaageeshwariyanu",

Biranavara in Kalyana at "Shyamakrishna Sodhari Gowri Parameshwari Girija",

Sri Venkateshwara in Thodi at "Maarajanakam Madhavanamakam",

Shambho Mahadeva in Kamavardhini at "Shambho Mahadeva Sharanagathajana rakshaka",

Palinchu Kamakshi in Madhyamavathi at "Kanthamagu Peru Pondhithivi Kaarunyamurthivai jagamu",

Balagopala in Bhairavi at "Neela neeradha shareera dheerathara",

Pakkala Nilabadi in Karaharapriya at " Tanuvurce vandana mona ninchuchu nara"

Chakkani Raja in Karaharapriya at "Kantiki Sundarataramagu Rupamemu",

Rama Rama Gunaseema in Simhendramadhyamam at "Munimaanasa dhama Mrughamatha Sulalama",

Ninne Nammithi in Simhendramadhyamam at "Pannagendhra Sayana Pannagari Vahana",

Entara Nithana in Harikambodhi at "Sheshudu Shivuniki Bushudu Lakshmana",

Ksheenamai Thiruga in Mukhari at "Etijesina Jagannathudu Shiramuna"

Dasarathe Nee Runamu in Thodi at "Bhakti leni kavi"

"Saasaksha" in Pantuvarali at "Baamini samudhya shya mohana Padbanamba kamaladara nivara"