Melakartha is a collection of fundamental musical scales (ragas) in Carnatic music (South Indian classical music). Melakartha ragas are parent ragas (hence known as janaka ragas) from which other ragas may be generated. 

The 72 Melakartha ragas are split into 12 groups called chakrās, each containing 6 ragas. The ragas within the chakra differ only in the dhaivatam and nishadam notes (D and N). The name of each of the 12 chakras suggests their ordinal number as well.

Interactive visualization of 12 chakrās

• Indu stands for the moon, of which we have only one – hence it is the first chakra.
• Nētra means eyes, of which we have two – hence it is the second.
• Agni is the third chakra it indicates three kinds of Agni.(Dakshina, Ahavaniyam and gArhapatyam)So agni indicates 3rd Chakra.
• Vēda denoting four Vedas is the name of the fourth chakra.
• Bāṇa comes fifth as it stands for the five Arrows of Manmatha.
• Rutu is the sixth chakra standing for the 6 seasons of Hindu calendar, which are Vasanta, Greeshma, Varsha, Sharat, Hemanta and Shishira.
• Rishi, meaning sage, is the seventh chakra representing the seven sages.
• Vasu stands for the eight vasus of Hinduism.
• Brahma comes next of which there are 9.
• Disi Chakra indicates Ten directions(East, West, North, South, North East, North West, South East, South West, Above and Below). Hence it is 10th Chakra.
• Eleventh chakra is Rudra of which there are eleven.
• Twelfth comes Aditya of which there are twelve.

Source: Wikipedia