In the course of past centuries the ceremonies of the Holy Mass organically grew to what have now in the 1962 Solemn High Mass, i.e. the High Mass with deacon and subdeacon.
When multiple Masses were to be celebrated, the ceremonies were simplified and abridged to what we call today the Low Mass. Here the priest (with only one altar server) had to do and say everything, including those parts that normally were being done and sung by the other Sacred Ministers. Since there was no choir present nor any deacon or subdeacon, it was decided that what was normally to sung in the Solemn High Mass, is being prayed up loud in the Low Mass, and what was said during, the priest now prays quietly.
However the Low Mass ended up influencing the Solemn High Mass as it is celebrated today. That is why some prayers take place twice: They are sung by the assistants and prayed in silence by the priest.
An example of this duplication of prayers is the Introit. The ‘schola’ (choir), since they are also to be considered some type of altar servers), sing the Introit during the start of the Mass. And yet the priest reads the Introit silently after the prayers at the foot of the altar.
Another example is the Epistle. The subdeacon sings the Epistle up loud, while the priest silently reads the same on the Epistle side of the altar.
(Source: De Mis, Van Beukering F., 1939)
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