"Do Popes Quit?" by Daniel Wakin, New York Times

http://www.nytimes.com/2010/04/11/weekinreview/11wakin.html

Do Popes Quit?
By DANIEL J. WAKIN
Published: April 9, 2010


"Of course, popes have resigned before — the last a mere 595 years ago, when Gregory XII stepped down to heal a schism. Before that, Celestine V, a fiercely ascetic former hermit who wore his temporal power heavily, resigned in 1294 (Dante consigned him to hell for cowardice, some interpreters of the “Inferno” believe)...

...John Paul was responsible for two recent but fleeting references to papal resignation in official church policy. A revision of the code of canon law issued under him, in 1983, says, “If it happens that the Roman pontiff resigns his office, it is required for validity that the resignation is made freely and properly manifested but not that it is accepted by anyone.”