As the requirements of the Roman liturgy expanded through the provision of new feasts with their proper texts it became necessary to supply them with music. The simplest way of doing this was to adapt existing melodies to the purpose. The earliest example of this procedure in the context of the Mass is to be found in the Proper of the Mass for Trinity Sunday. Trinity Sunday is simply the first Sunday after Pentecost, which like the other three Sundays following an Ember Week originally had no Mass of its own. In this case the gap was filled up by the votive Mass of the Holy Trinity; but in some places an alternative procedure, whereby the Masses of the succeeding Sundays were moved back one week en bloc, had already been adopted. This resulted in the confusing situation found in the modern Roman Gradual, in which the now superfluous Mass for the first Sunday after Pentecost is retained for use on the first vacant feria (GR, p. 311). The text of the Mass of the Holy Trinity is first met with around 850; but the earliest musical source, in which the Mass now heads the series of Sundays after Pentecost, is the tenth-century Gradual, St Gall 339. The sources from which the musical adaptations were made have been summarized by Apel (Gregorian Chant, p. 68).