Sometimes, however, it has happened through abdication (whether for personal or political reasons). And because succession is through a collateral line, potential complications can arise (what if the abdicating sovereign marries or remarries and fathers a son?). Although abdication is generally supposed to be a legal death, it sometimes is not understood or accepted as such. And when one is talking about the succession of an heir-presumptive, things can get really messy.
Now, there is nothing unusual about sovereigns abdicating per se: perhaps this has not been as common an occurrence as sibling succession, but it has happened (e.g. Holy Roman Emperor Charles V, to name just one example). Usually, however, the heirs who succeeded them have been CROWN princes. Traditionally throughout European royal history, such a person has been a king's eldest son and heir-apparent who expected to eventually inherit the throne, anyway.
Since succession is lateral, in such a scenario, there generally are no complications (e.g. the abdications of King Ludwig I of Bavaria in 1848, Tsar Ferdinand of the Bulgarians in 1948, King Leopold III of the Belgians in 1950, King Albert II of the Belgians in 2013, and King Juan Carlos of Spain in 2014).
But when an heir-presumptive succeeds, complications can arise, for reasons that a "nearer heir" could theoretically arrive on the scene, after abdication -- notwithstanding legal death. I believe this was the reason that special legislation had to be passed in the UK, to ensure that such a thing wouldn't happen in the case of King Edward VIII, who abdicated the throne in favor of his brother. What if, for instance, Wallis had suddenly died and been unable to marry him, or died suddenly after she married him, and he married or remarried with a more *suitable* woman who would have been more acceptable as queen? Or at least enjoyed a place in the British royal court with the qualification of Royal Highness (in addition to the title Duchess of Windsor), instead of being obliged to live in a self-imposed exile with her husband? Yes, I'm aware of the Letters Patent issues, to deny his wife the three letters HRH.
What if King Baudouin of the Belgians had abdicated in favor of his brother or nephew, back in the 1970's, only to have Queen Fabiola suddenly get pregnant and give birth to a son (theoretically possible, since she was still within childbearing age)? What if the widowed King Carlo Emanuele IV Sardinia had remarried and fathered a son, after abdicating the throne in 1802 in favor of his brother? What if the abdicated Grand Duchess Marie Adelaide of Luxembourg had decided to marry, and managed to give birth to a son?
Bearing all this in mind, I've been wondering if somebody could provide a list of sovereigns who abdicated their thrones in favors of heirs-presumptive, not heirs-apparent. Also, I'd like to know about the special acts of legislation which may have had to be passed, to ensure that the thrones remained in the hands of successors.