The word Purvanga means the first part or first section and the word Uttaranga means next part or second section of a scale.
Any octave (saptak) has 2 parts: Sa to Ma and Pa to Sȧ. These are also known as upper and lower tetrachords. The vadi and samvadi swar of a raga will never dwell in one single tetrachord ie if the vadi swar is in the first section, the samvadi swar will be in the second section and vice versa. A raga whose vadi swar is in the first section is called Purvanga Raga and if the vadi swar is in the second section, it is called Uttaranga Raga.
According to rules,Purvanga ragas are to be sung within the period of 1noon to midnight and the Uttaranga ragas are to be sung from midnight to noon. Majority of the ragas follow this rule, however there are a few exceptions.
Example of Purvanga Raga: Raga Yaman.
Raga Yaman has Gandhar as the vadi swar and Nishad as the samvadi swar.Therefore, most of the phrases of alaap or vistar are developed and unfolded within the orbit of the first part or Purvanga. It is therefore a Purvanga raga. Also raga Yaman is performed around 8 pm thereby satisfying the rule of Purvanga ragas being sung between noon and midnight.
Example of Uttaranga Raga: Raga Bhairav
Raga Bhairav has Dhaiwat as the vadi swar and Rishabh as the samvadi swar. Therefore, most of the phrases of alaap or vistar are developed and unfolded within the orbit of the second part or Uttaranga. It is therefore an Uttaranga raga. Also raga Bhairav is performed around 6am thereby satisfying the rule of Uttaranga ragas being sung after midnight and before noon.
In some ragas when both the vadi and samvadi swars appear to be within any one of the above tetrachords, the length of both the sections are interchanged and one of the swars is considered to dwell in the other tetrachord. In other words, the orbit of each section is enlarged to one full tone beyond its original boundary ie the orbit of the first section now extends from Sa to Pa and that of the second section from Ma to Sȧ, which was originally from Sa to Ma and Pa to Sȧ respectively. This extension has been made recently with a view to classify the ragas under above mentioned rules.
Example Raga Bhimpalasi:
Aroha: Sa Ga Ma Pa Ni Sȧ ; Avaroha: Sȧ Ni Dha Pa Ma Ga Re Sa
Here the vadi swar is Madhyam and is in the Purvanga (first section). Samvadi is considered as upper Shadja which falls within the Uttaranga (second section).
Hence raga Bhimpalasi is a Purvanga raga and as is sung at 5pm (after noon and before midnight) thereby compiling with the rules.