Yemen is dominated by several tribal confederations. Each includes tribes, clans and extended families.
Determining which is the most powerful is difficult because of intermarriage and shifting alliances.
For the most part, the confederations are located in the north. Before uniting with the north in 1990, South Yemen's socialist leaders reduced the power of the tribes.
Listed in order of influence, the confederations are:
Hashid. This is the second largest in population, with hundreds of thousands of followers.
Its leader is Sadeq Abdullah Al Ahmar and influential members include his brother, the prominent businessman and opposition leader Hamid Al Ahmar, plus another brother, Himyar Al Ahmar, the former deputy speaker of parliament.
The confederation is concentrated around the north-east governorate of Amran and associated tribes and clans include Al Osaimat, Othar, Kharef, Bani Suraim, Hamdan and Sanhan, which is the clan of the former president, Ali Abdullah Saleh.
Bakil. This has the largest population, with hundreds of thousands of followers.
Its leaders are from Abu Lahoum and Nihm tribes and it is concentrated north of Sanaa.
Associated tribes and clans include the Khawlan, Arhab, Al Hada, Al Jidaan, Anis, Dihm, Bani Mata and Al Haimatyeen.
Madhaj. This confederation is geographically dispersed but concentrated in the central part of the country.
It is known for its business professionals and educated elites, and associated tribes and clans include the Murad, Abidah, Ans, Al Zaraniq, Al Awaliq, Kaifah, Al Bakzm, Al Sabyha, Al Abadil, Al Alhasani and Al Fadhli.