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What is Mangal Dosha (Manglik)?
In short
What is Mangal Dosha?
A chart is called Manglik when Mars occupies the 1st, 2nd, 4th, 7th, 8th or 12th house. It is usually checked from the lagna, and also from the Moon and from Venus. The concern is traditionally about friction in marriage, attributed to Mars's association with heat, assertion and conflict.
The six houses, and three places to count from
The afflicted houses are the 1st, 2nd, 4th, 7th, 8th and 12th. These are the houses connected with the self, the family, domestic peace, marriage, longevity and loss — which is the logic behind the selection.
Practitioners differ on the reference point. Counting from the lagna is standard; many also count from the Moon, and a smaller number from Venus, the karaka of marriage. A chart Manglik from one reference and not from others is generally treated as mild.
Cancellation
Several cancellations are widely accepted: Mars in its own rashi (Mesha or Vrishchika) or exalted in Makara; Mars aspected by or conjunct Jupiter; both people being Manglik, which is the most commonly cited remedy of all; and — in some traditions — the dosha weakening with age.
Manglik status is one factor. A chart is not a verdict, and a calculator that returns a yes or no has told you where Mars is, not what a marriage will be.