Article 4 of the Kuwait constitution stipulates that Kuwait is a hereditary emirate whose emir must be an heir of Mubarak Al-Sabah. Mubarak had four sons, but an informal pattern of alternation between the descendants of his sons Jabir and Salem emerged since his death in 1915. This pattern of succession had one exception before 2006, when Sheikh Sabah Al-Salim, a son of Salem, was named crown prince to succeed his half-brother Sheikh Abdullah Al-Salem as a consequence of infighting and lack of consensus within the ruling family council. The alternating system was resumed when Sheikh Sabah Al-Salim named Sheikh Jaber Al-Ahmed of the Jabir branch as his crown prince, eventually ruling as Emir for 29 years from 1977 to 2006. On January 15 2006, Emir Sheikh Jaber Al-Ahmed died and his crown prince, Sheikh Saad Al-Abdullah of the Salem branch was named Emir. On January 23 2006, the National Assembly unanimously voted in favor of Sheikh Saad Al-Abdullah abdicating in favor of Sheikh Sabah Al-Ahmed, citing his illness with a form of dementia. Instead of naming a successor from the Salem branch as per convention, Sheikh Sabah Al-Ahmed named his half-brother Sheikh Nawaf Al-Ahmed as crown prince and his nephew Sheikh Nasser Al-Mohammed as prime minister.