There are 7 basic notes or swars (Shadaj, Rishabh, Gandhar, Madhyam, Pancham, Dhaivat, Nishad) in the Hindustani Classical Music, which are called as Shuddha (pure) swars. Out of these swars Rishabh, Gandhar, Dhaivat and Nishad can be moved one note below and then they are called as komal (soft or flat) swars. So now there are 7 shuddha swars + 4 komal swars = 11 swars. Again, Madhyam can be moved one note above and then it is called as teevra (sharp) swar. So now there are 7 shuddha swars + 4 komal swars + 1 teevra swar = 12 swars. Thus hindustani classical music consists of total 12 swars. These 12 swars may be broadly classified as achalit (immovable) and chalit or vikrut (movable). Achalit swars are Shadaj and Pancham. The rest 10 swars are all chalit swars.
Saptak is a scale or register of 7 notes (Sa, Re, Ga, Ma, Pa, Dha, Ni). Hindustani classical music is formed within three saptaks namely, Mandra Saptak, Maddhya Saptak, and Taar Saptak. Human voice is neither too high nor too low in its scale and thus this middle scale is known as Maddhya Saptak. This is the normal scale that is most frequently used in singing. With reference to this Maddhya saptak, the lower scale is known as Mandra or Kharaj Saptak and the higher scale is known as Taar Saptak. Mandra saptak has half the frequency of the Maddhya Saptak and Taar Saptak has twice the frequency of the Maddhya Saptak. Thus the 7 notes of Mandra Saptak are lower than the 7 notes of the Maddhya Saptak, while the 7 notes of the Taar Saptak are higher than the 7 notes of the Maddhya Saptak. In instrumental music it is possible to go beyond these scales. When the notes go beyond the Mandra Saptak it is known as Ati Mandra Saptak and when the notes go beyond the Taar Saptak it is known as Ati Taar Saptak.
The harmonium has these three saptaks as its keys. The keys are either white (saphed) or black (kali). From the left, the mandra saptak is the first one, then comes the maddhya saptak and thereafter the taar saptak. The first saphed (white) key of the harmonium from the left is the Shadja (Sa) of the mandra saptak. The succeeding white key is Rishabh (Re) and so on the first seven successive white keys form the mandra saptak. Thereafter the succeeding 7 white keys form the maddhya saptak and thereafter the 7 succeeding white keys form the taar saptak. The kali (black) keys usually play the sharp notes. In the maddhya saptak, the first white key (Pehli Saphed) is equivalent to C of the Western Scale. Similarly, the second white key (Doosri Saphed) is equivalent to D of the Western Scale. The kali (black) key present between the first and second white key is known as first black key (Pehli Kali) and is equivalent to C Sharp of the Western scale. Thus, the Western equivalent of the Indian scale is as follows:
First White (Pehli Saphed) key of Madhya Saptak is equal to C of Western Scale.
First Black (Pehli Kali) = C Sharp
Second White (Doosri Saphed) = D
Second Black (Doosri Kali) = D Sharp
Third White (Teesri Saphed) = E
Fourth White (Chouthi Saphed) = F
Third Black (Teesri Kali) = F Sharp
Fifth White (Panchvi Saphed) = G
Fourth Black (Chouthi Kali) = G Sharp
Sixth White (Chhathi Saphed) = A
Fifth Black (Panchvi Kali) = A Sharp
Seventh White (Saatvi Saphed) = B
Normally males sing in C, D and E whereas females sing in F and G scales. However, it is important to understand that any note can become Shadja or Sa and a saptak can be built successively from that note onwards. Thus, as per the desire and capacity of the singer, the saptak can start from any key of the harmonium.