Nadi Astrology (Nadi jyothidam) is a form of Dharma astrology practiced in Tamil Nadu, Kerala and adjacent regions in India. Basically, the Nadi palm leaves are located based on the thumb impressions (right for men, left for women). There are 150 Nadi's in a sign; each sign is 30 degrees of the zodiac 360. Twelve signs of zodiac are grouped into three categories: Movable (Chara), Fixed (Sthira) and Dual (Dvisvabhava) signs. The phraseology of 150 Nadis is distinctive to each of these three types of signs. There are 1,800 Nadis in 360 degrees. Numbers and names of Nadi's are similar among all four Chara signs. Among all four Sthira signs, numbers and names of Nādis are alike, but numbering is peculiar from those in Chara and Dvisvabhava signs. Further, numbering of Nadi's in all four Dvisvabhava signs are similar among themselves, but different from Chara or Sthira signs.
Nadi texts utilize the concept of Nadi as the fundamental unit for foresight. That is why they are called "Nadi amshas". Nadi leaves were not only utilized for herbal and medicinal purposes but also used for future prediction. The foredoom written on the leaf is in form of poems in ancient Tamil, same as the language used in ancient temple of Tamilnadu.
The fundamental procedure of these Nadi or Samhita scripts have never been illustrated in these texts, hence astrologers are restricted to predict only what they already retained, and are dumb-founded when they do not possess the needed leaf about some specific person.
When predictions are based on Nadi's in which ancestors and planets are placed; it refers the combined finalization of all sixteen divisional charts are being told. Hence, Nadi Astrology is the most explained and accurate method. But it requires highly exact birth time, and it also necessitates knowledge of genuine Nadi texts, most of which are undeclared, and the guardians do not allow others to see the manuscripts. The same is the case with Samhita scripts in North India, such as Bhrigu Samhita or Ravana Samhita.