The apostolic letter given motu proprio on the use of the Roman liturgy prior to the reform of 1970, issued with a letter to the bishops of the world explaining that it followed a consultation of the episcopate about the effects of Summorum Pontificum. In eight articles it declares that the liturgical books promulgated by Paul VI and John Paul II are the unique expression of the lex orandi of the Roman Rite, makes the diocesan bishop the sole competent authority to authorise the use of the 1962 Missal in his diocese, restricts where and by whom it may be celebrated, and directs that readings be proclaimed in the vernacular.