65. In sung or said Masses, the organ, or other instrument legitimately admitted, can be used to accompany the singing of the choir and the people; it can also be played solo at the beginning before the priest reaches the altar, at the Offertory, at the Communion, and at the end of Mass.
The only time the GIRM says not to play the organ is when the priest is chanting. It "goes without saying" that we can just play the Bach Allein Gott trio instead of the Gloria!
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