SAPTAKA

The Saptaka
On the keyboard, the area S through N is called a saptaka (an octave). There are three octaves: the madhya (middle), the mandra (lower), and the taar (higher/upper). The lower octave is situated to the left of the middle; the upper octave is situated to the right of the middle octave. Again, this is clearly shown in the diagram.
A note belonging to the middle octave has no sign when represented on the paper; for example, the P of the middle octave is represented simply as: P. The note belonging to the lower octave has an apostrophe to its LEFT. Thus, for example, the P in the lower octave is represented as: 'P. The note belonging to the higher octave has an apostrophe to its RIGHT. Thus, for example, the P in the higher octave is represented as: P'. Thus, the 36 keys represent the notes in all three octaves:

'S, 'r, 'R, 'g, 'G, 'm, 'M, 'P, 'd, 'D, 'n, 'N

S, r, R, g, G, m, M, P, d, D, n, N

S', r', R', g', G', m', M', P', d', D', n', N'.
To summarize:
§ Shuddh (natural) notes are notated as S, R, G, m, P, D, N
§ Komal (flat) notes are notated as r, g, d, n
§ Teevra (sharp) Ma is notated as M
All notes belong to madhya-saptak by default; they have no sign. Notes of mandra-saptak are preceded by ' sign, and notes of taar-saptak are succeeded by ' sign.

Lastly, a comma (,) represents a pause. A dash (-) is used to avoid any confusion or misunderstanding.