This is what the General Instruction says: 177. New hymns can be set to traditional melodies of the same rhythm and meter.
178. For vernacular celebration, the conferences of bishops may adapt the Latin hymns to suit the character of their own language and introduce fresh compositions, [13] provided these are in complete harmony with the spirit of the hour, season, or feast. Great care must be taken not to allow popular songs that have no artistic merit and are not in keeping with the dignity of the liturgy.
Does anyone have any "translation" of what was intended for 177?
How did 178 mean we can choose any hymn for an office we deem appropriate?
I think most probably use the wishy-washy instruction at 42 under Lauds and Vespers as the justification for just singing whatever hymn you want.
A suitable hymn is then said. The hymn should be composed so as to express the particular characteristic of each Hour or feast. It makes an easy and pleasant opening to the prayer, especially in celebrations with the people.
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