In the letter, Pope Benedict explains the positive motivation behind his decision as “a matter of coming to an interior reconciliation in the heart of the Church.”
Reconciling who? In short:
- those in the Church who never fully accepted Vatican II, its “Constitution on the Liturgy” and its openness to “update” the Liturgy, and
- those who mistook the new openness for a “rupture” with the past and never bothered to read the “Constitution on the Liturgy” and/or completely ignore its instructions to foster education of the Sacred Music tradition of the Church, and ignored even more its importance to Church unity in post-Vatican II liturgies.
The “mutual enrichment” that results from having two forms of the rite has recently been re-affirmed as the path to Church unity by the Prefect of the Congregation for Divine Worship appointed by Pope Francis, Cardinal Sarah (download article: True liturgy is a reflection of heaven ).
To explore the documents regulating the musical celebration of the Extraordinary Form of the Roman Rite (Tridentine Mass), a good place to start are pages 9 to 15 of this document : Psallite-Sapienter-A-Musicians-Guide-to-the-Extraordinary-Form-2
Also these 1958 instructions: de musica sacra et sacra liturgia
If the priest does not chant in a Tridentine Mass, then we have what is called “Low Mass”. The rules for singing are then different: Music-for-low-mass
See also Books, books, books,…
Example of a practical application: join the Soli Deo Gloria schola.