1991 0-7734-9960-1 Discusses custom as a source of law. After tracing its basis in Roman law, shows its development in both the Eastern and Western Church. Emphasizes its stabilizing influence, evident in the work of Isidore of Seville, Yves of Chartres, and Gratian. The Decretists subordinated custom to law, a position maintained by Gregory IX in his Decretals (1234). The later history illustrates the shift from the "reasonable custom" to the "will of the competent superior." Shows the need of the Church to recover Custom in order to promote missionary activity, ecumenical discourse within the contest of pluralism.