A pope can resign at any time.
The BBC's David Willey in Rome says the move has come as a shock - but adds that in theory there has never been anything stopping Pope Benedict or any of his predecessors from stepping aside.
Under the Catholic Church's governing code, Canon Law, the only conditions for the validity of such a resignation are that it be made freely and be properly published.
But resignation is extremely rare: the last Pope to step aside was Pope Gregory XII, who resigned in 1415 amid a schism within the Church.
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