Poorvanga and Uttaranga

An anga in general means a sub-division, a part of the whole.

In Indian music, it denotes one of the two parts (tetrachords) of the octave.

First, to the saptaka (S, R, G, m, P, D, N), add the taara Sa (S'). This is the octave (S, R, G, m, P, D, N, S'). Now the octave is divided into two parts:
(1) Sa to Ma: S, R, G, m;
(2) P to S': P, D, N, S'.
These constitute the two anga-s. The first part is called poorvanga, the lower tetrachord. The second part is called uttaranga, the upper tetrachord. Each part has four members, and the two parts are similarly constructed.