Carl XVI Gustaf became the king of Sweden after the death of his grandfather, King Gustaf VI Adolf, in September of 1973, as Carl's father, Prince Gustaf Adolf, Duke of Västerbotten, died in an aeroplane crash in Denmark when Carl was only 9 months old. At 72 years old, Carl is the longest-reigning monarch in Swedish history, having surpassed King Magnus IV's reign of 44 years in April. Though his son, Prince Carl Philip, was briefly the heir apparent, a new law establishing absolute primogeniture passed less than a year after Philip's birth, making his oldest sister, Crown Princess Victoria, next in the line of succession.