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In 1688, the Catholic king of England and Scotland, James II/VII of the House of Stuart, was deposed and replaced with Protestant co-monarchs William III and Mary II in what is known as the "Glorious Revolution". They were followed by Mary's younger sister Queen Anne and then upon her death, the throne passed to a second cousin living in Germany. That individual became King George I and with him, the throne of Great Britain passed to the House of Hanover.
But what if the line had continued through James II/VII's son instead? This chart shows the alternative kings and queens of Great Britain that theoretical could have existed if history had taken a different path. Obviously, if the supporters of James II/VII and/or his descendants (known as Jacobites) had in fact been able to hold on to or reclaim the throne, there likely would have been different marriages and hence different children and different titles. But if Scotland or Great Britain were to ever decide to legitimize the Jacobite succession (an extremely unlikely scenario), this chart shows who would be king today and in the near future.
The chart also shows how the future King William V (now prince William, Duke of Cambridge) will be the first descendent of King Charles I in over 300 years to sit on the throne.