Out of the ten poruthams prescribed by South Indian Astrological rules, it is called as "Mahendra Porutham".
Hindu Rashi/ Nakshatra Matching
In South India, Mahendra Matching (Porutham) of the bride and groom. It is mostly done when the marriages are arranged by the families of the boy and the girl.
What is the meaning of Mahendra porutham?
Mahendra porutham for marriage, is the one porutham that determines the strength of the girl's mangalyam [in archaic language], or the span of life together as man and wife.
When mahendra porutham compatibility is good, it is indicated that the resultant marriages will be long, not necessarily happy [these are determined by other factors, such as vasyam, yoni, stree dheergam etc], but a long one.
Mahendra porutham calculation
If the boy's star is 4th, 10th, 13th, 19th, 22nd from the girl's star, Mahendra porutham matching is good.
If counted from girl's star if you get 4,7,10,13,16,19,22,25 the match is uthamam. Otherwise no match.
Mahendra porutham - medium match If the boy's star is 7th,16th or 25th from the girl's, the porutham is only Medium.