king of Alba, the united kingdom of the Picts and Scots (858–862), and brother and successor of Kenneth I MacAlpin (Kenneth I). Donald established an ancient corpus of laws and rights (known as the laws of Aed, or Aedh) that apparently included the custom of tanistry. According to this custom, the successor of a king was elected during his lifetime from the eldest and worthiest of his kin, often a collateral (brother or cousin) in preference to a descendant (son). The next king, Donald's brother Constantine I, succeeded in accordance with this custom.