In Tibet, polyandry (marriage of one woman to more than one husband) was accepted. Brothers would marry one woman, and the children of the marriage would inherit the land. The term for polyandry where the husbands are brothers to each other is fraternal polyandry.
Like many cultural customs, polyandry in Tibet was compatible with specific challenges of geography. In a country where there was little tillable land, the practice of polyandry would reduce the number of heirs, because a woman has more biological limits on the number of children she can have, than a man does. Thus, the land would stay within the same family, undivided. The marriage of brothers to the same woman would ensure that brothers stayed on the land together to work that land, providing for more adult male labor. The practice would also ensure that if one husband needed to travel -- for instance, for trade purposes -- another husband would remain with the family and land.
Polyandry is now against the law in Tibet, though it is occasionally still practiced.