The term "Sadharana" means "being common to two or more elements".
Sadhara?a is of two kinds:
In the ancient musical practice only two of the seven basic notes could be altered, i.e. the minor third (ga) and the minor seventh (ni), which could respectively be changed into the major third (antara ga) and the major seventh (kakali ni). The altered notes were used as passing or alternating notes. They figured in modes (jati) in which the notes ga and ni did not function as a dominant (amsa).
For example, in Sadjagrama murchana, in the four sruti interval existing between sadja and nisada, the additional svara, kakali nisada can occur. The position of kakali nisada would be two srutis higher than the sadjagrama-nisada and two srutis lesser than the position of sadja.
It is considered as a variety of nisada and not sadja because kakali nisada occurs only when the grama nisada is a weak note.
In the same way, another sadharana svara called "antara gandhara" occurs in between gandhara and madhyama. This gandhara is two srutis higher than the grama gandhara and two srutis less than madhyama.
The two Sadharana svaras kakali nisada and antara gandhara occur in both the gramas.
According to Natyasastra these two cases of overlapping may also be regarded as one case, if madhyamagrama is construed by renaming the notes of sadjagrama. Antara ga of the transposed madhyamagrama may be called kakali ni in sadjagrama.
Since this overlapping of intervals (svara sadharana) could be explained in two basic scales (grama), it was also regarded as grama sadharana.
According to Abhinavagupta the sadharana procedure also called kaisika, involved the following notes:
In the first chapter "Svaragatadhyaya" of the Sangitaratnakara, after discussing sruti, the author speaks of svaras and lists suddha (literary "pure") and vikrita (literary "modified") svaras.
The suddha and vikrita svaras mentioned in Sangitaratnakara (I.3.39-46) are listed below:
The svaras are shown below in accordance with the positions they occupy in the 22-sruti-scale:
|Suddha svara||Suddha sadja||Suddha risabha||Suddha gandhara|
|Vikrita svara||Kaisika nisada||Kakali nisada||Cyuta sadja||Acyuta sadja||Vikrita-catusruti risabha||Sadharana gandhara||Antara gandhara|
|Suddha svara||Suddha madhyama||Suddha panchama||Suddha dhaivata||Suddha nisada|
|Vikrita svara||Cyuta Madhyama||Acyuta Madhyama||
In the above list we see that the suddha-svaras are those which are located on the positions occupied by the svaras of the sadjagrama.
We also infer that those svaras which differ from the sadjagrama svaras with regard to either their positions of interval are termed vikrita svaras.
The following points are to be noted regarding these.