[Just mythoughts]

I have no recollection of the Catholic in-laws ever referencing the ‘key of David’... the slumber continues.

[vladimir998]

https://www.catholicmom.com/articles/2014/12/08/why-do-we-call-jesus-key-of-david

[Hieronymus]

I have no recollection of the Catholic in-laws ever referencing the ‘key of David’...

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First line, third verse—sung every Advent since the 9th century at the latest, predating English by a good bit,

Veni, veni Emmanuel;
Captivum solve Israel,
Qui gemit in exilio,
Privatus Dei Filio.

Refrain
Gaude! Gaude! Emmanuel,
Nascetur pro te, Israel!

Veni, veni, O Oriens;
Solare nos adveniens,
Noctis depelle nebulas,
Dirasque noctis tenebras.
Refrain

Veni, Clavis Davidica!
Regna reclude caelica;
Fac iter tutum superum,
Et claude vias inferum.
Refrain