In the basic details of a birth chart, there is a classification of type NADI.
There are three types of Nadi: Adya, Madhya, and Antya.
What do this Nadi and the three types signify? Which scripture says about it?
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Already one answer is posted. So I am using a different source to answer.
From the Vivaha Prakarana chapter of the book "Essence of Dharma Sindhu":
The Nakshatras viz. Ashwini, Ardra, Punarvasu, Uttara, Hasta, Jyeshtha, Moola, Shatabhishak, and Purvaabhaadra constitute ‘ Prathama Naadi’; Bharani, Mrigaseersha, Pushya, Purva Phalguni , Chitra, Anuradha, Purvaashaadha, Dhanishtha and Uttara- bhadra form the Madhyama Naadi and Krittika, Rohini, Asresha, Magha, Swaati,Vishakha, Uttaraashaadha, Shravana and Revati are in the Charama Nadi or in th Last category.If both the Vadhu Varas have the same Naadi, then the result could even by Mrityu! Difference in Naadis enjoys the maximum of eight Gunas.
Before marriage it is to be ensured that the bride and groom don't have the same Nadi. It is one of the many matching tasks that traditionally astrologers perform before marriage.
Nadi Jyotish applied for this purpose is called Nadi Milan (joining) and is used for matching horoscopes of a boy and girl for potential marriage. They should be of different nadis.
If Moon is placed in any of these constellations named Ashwini, Ardra, Punarvasu, Hasta, Jyeshtha, Moola, Uttara Phalguni, Shatabhisha or Poorvabhadrapada in birth chart, the person is said to have Aadi Nadi.
If Moon is placed in any of these constellations named Bharani, Pushya, Mrigashira, Poorvaphalguni, Chitra, Poorvashada, Dhanishtha, Anuradha, or Uttarabhadrapada in birth chart, the person is said to have Madhya Nadi.
If Moon is placed in any of these constellations named Kritika, Ashlesha, Magha, Rohini, Swati, Uttarashada, Vishakha, Revati or Shravana in birth chart the person is said to have Antya Nadi.
According to Ayurveda, the three different Nadi Adya, Madhya and Antya represent Vaat, Pita and Kapah that is body constitution.
The significance is that if the man and woman are from the same Nadi, they can not be married to each other.
Sources: Ancient scriptures of Piyush Dhara and Hora Shastra taken from the book TEXT BOOK OF SCIENTIFIC HINDU ASTROLOGY, in two volumes by P.S. Sastri,