The 'natural' scale is the scale which produces natural sounds which are naturally consonant with the Shadja and are pleasing to the ear.
When we hear an extremely well-tuned Tanpura, we find that it makes 3 such fundamental sounds.
Firstly, the 'Shadja' in which it is tuned, it's 'Pancham', and the natural 'Gandhar'. These 3 notes when taken serially and their frequencies analysed, Shadja, Gandhar, and Pancham are found to be situated at a fixed distance.
If Shadja is of 100 Hz, Gandhar is at 125 Hz, and Pancham at 150 Hz. How these natural harmonics/frequencies evolve has already been shown in Topic 4.
S:G:P are therefore the basic 3 universally accepted natural musical notes and they give rise to the 7 natural notes as shown below. (Refer to following table)
This system gives us 3 perfectly consonant triads at a ratio of 100:125:150 consisting of S-G-P, M-D-S', and P-N-R'.