The apostolic letter given motu proprio on the use of the Roman liturgy prior to the reform of 1970, accompanied by a letter to the bishops of the world explaining the pope's reasons. It declared that the Roman Missal promulgated by Paul VI is the ordinary expression of the lex orandi of the Roman Rite and that the Missal of 1962 is to be regarded as an extraordinary expression of the same lex orandi, never abrogated; it permitted priests to celebrate with the older Missal without special permission and set out how stable groups of the faithful might request it in parishes.