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stephraim
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The General Instruction of the Roman Missal allows for all the propers (Introit, Gradual, Alleluia/Tract, Offertory, and Communio) and all the ordinary (Kyrie, Gloria, Credo, Sanctus, Agnus Dei) to be sung in Gregorian chant using the Graduale Romanum or Graduale Simplex. Further, the “alius cantus aptus” option allows for other Gregorian chant books, such as the Cantus Selecti, to be used as well. So, a schola is allowed to sing all those parts. The dialogues may be sung in Latin as well, though preferably all the assembly should sing.