Charles the Great and Otto the Great both called councils in Rome to try Popes. But now the Pope has attained such a pre-eminence that he cites the Emperor to appear before him to justify his conduct. Verily the Papacy, with the aid of Damiani and Hildebrand, had got out of the quagmire which almost engulfed it in the tenth and the eleventh centuries. At the same time the imperial right to choose Popes, which had so long been exercised and which had been recognised again and again by the Popes themselves, was taken out of the Emperor's hands and entirely controlled by the Roman cardinals. SOURCES A.--PRIMARY: 1.--Henderson, E. F., _Select Historical Documents of the Middle Ages_. N. Y., 1892. 2.--Gee, H., and Hardy, W. J., _Documents Illustrative of English Church History_. Lond., 1896. 3.--Neale, J. M., _Mediæval Preachers_. Lond., 1856. 4.--Thatcher and McNeal, _Source Book for Mediæval History_. N. Y., 1905. Bibliographical Note:--The primary material for this subject is practically all in Latin. The most valuable collections are: Migne, _Patrologia_, vols. 119-145; Pertz, _Monumenta_; Mansi, _Sacrorum Conciliorum_; _Rolls Series_; Muratori; _Bouquet_. B.--SECONDARY: I.--SPECIAL: 1.--Baring-Gould, S., _Lives of the Saints_. Lond., 1897-8. 15 vols. 2.--Bowden, J. W., _Life and Pontificate of Gregory VII._ 2 vols. Lond., 1840. I., 73-283. 3.--Bryce, J., _The Holy Roman Empire_. Various eds. Rev. ed., 1904. 4.--Butler, A., _Lives of the Fathers, Martyrs, and Saints_. Dub., 1866. 12 vols. 5.--Fisher, H., _The Mediæval Empire_. 6.--Greenwood, A., _The Empire and the Papacy in the Middle Ages_. 7.--Greenwood, T., _Cathedra Petri_. Ch. 4. 8.--Greisley, Sir R., _Life and Pontificate of Gregory VII._ Lond., 1832. Introduction. 9.--Lea, H. C., _History of Sacerdotal Celibacy_. Rev. ed. 10.--Maitland, S. R., _The Dark Ages_. Lond., 1889. 11.--Montalembert, Count de, _The Monks of the West_. Lond., 1896. 7 vols. 12.--Stephens, W. R. W., _Hildebrand and His Times_. N. Y., 1888. 13.--Villemain, A. F., _Life of Gregory VII._ Lond., 1874. 2 vols. 14.--Vincent, M. R., _The Age of Hildebrand_. N.Y., 1896. II.--GENERAL: