In the middle of the 14th century, foreign minstrels and chanters in the service of the royal house of Catalonia-Aragon introduced new musical styles into the country. During the reigns of Peter IV of Aragon (Peter III in the principality of Catalonia, reigned 1336−87), John I (reigned 1387−95), and Martin I (called Martin the Humane, reigned 1396−1410), most of the minstrels came from Germany, Flanders, France, England, Italy, and Castile. The monarchs of the royal house of Catalonia-Aragon were considered among the most outstanding patrons in the kingdom, and the royal court became one of the most important music centers in Europe. The Ars Nova, the musical style of the time, played a very important role in the court of Aragon, contrary to what happened in the rest of the Iberian Peninsula. One reason for this was the admiration that the courts of John I and Martin the Humane had for French musicians, especially Guillaume de Machaut (circa 1300−1377). In addition, many of the singers at the royal chapel in Barcelona came from the pontifical chapel of Avignon, so the former became a sort of subsidiary institution of the latter. This polyphonic mass from the end of the 14th century is part of the liturgical repertoire of the time. It is one of the first complete polyphonic cycles of the ordinary part of the mass that has remained in use until the present, similar, for example, to Guillaume de Machaut’s Mass of Notre-Dame. It comes from the repertoire of Avignon and is linked with the Catalan and Aragonese musical chapel. It contains the usual parts of the Mass: Kyrie, Gloria, Credo, Sanctus, and Agnus Dei and is written in three voices except for the last part, the Agnus Dei, which is in four voices. Although it maintains internal consistency, different parts were composed independently and it was not until later that they were joined to form the mass. They belong to different composers, most of them anonymous. The Credo is attributed to Sortes or Sortis, who may be identified as Steve de Sort, musician of the court of Aragon, or as Nicholes Sortes, who was related to Avignon. The Missa Barcelona is a small, beautiful manuscript on parchment that was added to the collections of the Biblioteca de Catalunya in 1926.