The Latin Mass Society has just added to its website a comprehensive guide to singers of when to sing what pieces of chant, and how long they have before they disrupt or delay the Mass.It includes the two videos I posted earlier, showing the sequence of events leading up to the singing of the Sanctus and to the Agnus Dei.
The guide, while not infallible, has been pored over by a number of singers and rubricists, and it is to be hoped that it will prove useful to singers less familiar with the Extraordinary Form (the Traditional Mass) when called upon to accompany it, and even those who wish to confirm their instincts. It increasingly happens that a parish choir is asked to provide singing for a traditional Requiem or a Mass for some special occasion, and while mastering the chants is one issue exactly what they are expected to do and when is quite another!
See the Guide here.
