THE EXTRAORDINARY FORM OF THE ROMAN RITE
The Roman Missal promulgated by Pope Paul VI is the ordinary expression of the lex orandi (rule of prayer) of the Catholic Church of the Latin rite. The Roman Missal promulgated by Saint Pius V and revised by Blessed John XXIII is nonetheless to be considered an extraordinary expression of the same lex orandi of the Church and duly honoured for its venerable and ancient usage. These two expressions of the Church’s lex orandi will in no way lead to a division in the Church’s lex credendi (rule of faith); for they are two usages of the one Roman rite. Pope Benedict XVI, Summorum Pontificum, art 1. Emphasis added.