King Charles III succeeded his mother Queen Elizabeth II on the British throne after her death on Sept. 8 last year, becoming king of not just the United Kingdom, but also Australia, Canada, New Zealand and 11 other countries. Under the British constitution, a sovereign succeeds to the throne the moment his or her predecessor dies, before being proclaimed to the people, so there is no interregnum. The rules, most of which date back hundreds of years, mean the crown passes to the monarch's eldest child, and the line is then dictated by birth order and closeness to the existing sovereign.