Polyandry refers to a form of marriage

Marriage

Marriage is a social union or legal contract between people that creates kinship. It is an institution in which interpersonal relationships, usually intimate and sexual, are acknowledged in a variety of ways, depending on the culture or subculture in which it is found...

 in which a woman has two or more husbands at the same time. The form of polyandry in which a woman is married to two or more brothers is known as "fraternal polyandry", and it is believed by many anthropologists to be the most frequently encountered form.
According to inscriptions describing the reforms of the Sumer

Sumer

Sumer was a civilization and historical region in southern Mesopotamia, modern Iraq during the Chalcolithic and Early Bronze Age....

ian king Urukagina

Urukagina

Urukagina , alternately rendered as Uruinimgina or Irikagina, was a ruler of the city-state Lagash in Mesopotamia...

 of Lagash

Lagash

Lagash is located northwest of the junction of the Euphrates and Tigris rivers and east of Uruk, about east of the modern town of Ash Shatrah. Lagash was one of the oldest cities of the Ancient Near East...

 (ca. 2300 BC), he is said to have abolished the former custom of polyandry in his country, on pain of the woman taking multiple husbands being stoned with rocks upon which her crime is written.

Polyandry in human relationships occurs or has occurred in Tibet

Polyandry in Tibet

Polyandry is a form of polygamy whereby a woman has several husbands. In Tibet those husbands are often brothers, which is why it is most commonly called "Fraternal Polyandry". Concern over the delicate question of which children are fathered by which brother falls on the wife alone...

; the Canadian Arctic; northern parts of Nepal

Nepal

Nepal , officially the Federal Democratic Republic of Nepal, is a landlocked sovereign state located in South Asia. It is located in the Himalayas and bordered to the north by the People's Republic of China, and to the south, east, and west by the Republic of India...

; Nigeria

Nigeria

Nigeria , officially the Federal Republic of Nigeria, is a federal constitutional republic comprising 36 states and its Federal Capital Territory, Abuja. The country is located in West Africa and shares land borders with the Republic of Benin in the west, Chad and Cameroon in the east, and Niger in...

; Bhutan

Bhutan

Bhutan , officially the Kingdom of Bhutan, is a landlocked state in South Asia, located at the eastern end of the Himalayas and bordered to the south, east and west by the Republic of India and to the north by the People's Republic of China...

; parts of India (Ladakh

Ladakh

Ladakh is a region of Jammu and Kashmir, the northernmost state of the Republic of India. It lies between the Kunlun mountain range in the north and the main Great Himalayas to the south, inhabited by people of Indo-Aryan and Tibetan descent...

; Zanskar

Zanskar

Zanskar is a subdistrict or tehsil of the Kargil district, which lies in the eastern half of the Indian state of Jammu and Kashmir. The administrative centre is Padum...

); the Nymba; Sri Lanka

Sri Lanka

Sri Lanka, officially the Democratic Socialist Republic of Sri Lanka is a country off the southern coast of the Indian subcontinent. Known until 1972 as Ceylon , Sri Lanka is an island surrounded by the Indian Ocean, the Gulf of Mannar and the Palk Strait, and lies in the vicinity of India and the...

; and some pre-contact Polynesian

Polynesians

The Polynesian peoples is a grouping of various ethnic groups that speak Polynesian languages, a branch of the Oceanic languages within the Austronesian languages, and inhabit Polynesia. They number approximately 1,500,000 people...

 societies, though probably only among higher caste women in this last.