Frederick I (Swedish: Fredrik I; 28 April 1676 5 April 1751) was prince consort of Sweden from 1718 to 1720, and King of Sweden from 1720 until his death and (as Frederick I) also Landgrave of Hesse-Kassel from 1730. He ascended the throne following the death of his brother-in-law absolutist Charles XII in the Great Northern War, and the abdication of his wife, Charles's sister and successor Ulrika Eleonora, after she had to relinquish most powers to the Riksdag of the Estates and thus chose to abdicate. His powerless reign and lack of legitimate heirs of his own saw his family's elimination from the line of succession after the parliamentary government dominated by pro-revanchist Hat Party politicians ventured into a war with Russia, which ended in defeat and the Russian tsarina Elizabeth getting Adolf Frederick of Holstein-Gottorp instated following the death of the king.

Ulrika Eleonora or Ulrica Eleanor (23 January 1688 – 24 November 1741), known as Ulrika Eleonora the Younger, was Queen of Sweden, reigning in her own right from 5 December 1718 until her abdication on 29 February 1720 in favour of her husband King Frederick, and then as his consort until her death.

She was the youngest child of King Charles XI and Ulrika Eleonora of Denmark and named after her mother. After the death of her brother King Charles XII in 1718, she claimed the throne. Her deceased older sister, Hedvig Sophia, had left a son, Charles Frederick of Holstein-Gottorp, who had the better claim by primogeniture. Ulrika Eleonora asserted that she was the closest surviving relative of the late king (the idea of proximity of blood) and cited the precedent of Queen Christina. She was recognized as successor by the Riksdag after she had agreed to renounce the powers of absolute monarchy established by her father. She abdicated in 1720 in favor of her husband, Landgrave Frederick I of Hesse-Kassel.