From the Apostolic Letter
In the form of “Motu Proprio”
Having pondered at length the pressing requests of these faithful to our Predecessor John Paul II, having also heard the Fathers of the Consistory of Cardinals held on 23 March 2006, having pondered all things, invoked the Holy Spirit and placed our confidence in the help of God, by this present Apostolic Letter we DECREE the following.
Art. 1. The Roman Missal promulgated by Paul VI is to be regarded as the ordinary expression of the law of prayer (lex orandi) of the Catholic Church of Latin Rite, while the Roman Missal promulgated by St Pius V and published again by Blessed John XXIII as the extraordinary expression of the law of prayer (lex orandi) and on account of its venerable and ancient use let it enjoy due honor. These two expressions of the law of prayer (lex orandi) of the Church in no way lead to a division in the law of prayer (lex orandi) of the Church, for they are two uses of the one Roman Rite.
Hence it is licit to celebrate the Sacrifice of the Mass in accordance with the typical edition of the Roman Missal promulgated by Blessed John XXIII in 1962 and never abrogated, as the extraordinary form of the Liturgy of the Church.