The name of Taverner's setting of the Kyrie known as the 'Leroy Kyrie' which you'll also see written as 'Kyrie Le Roy ' is generally believed to a reference to 'Roy Henry', either Henry IV or – more likely, Henry V to whom some musical works are attributed in the Old Hall Manuscript. Simply by virtue of having been composed it's quite an unusual piece of music for pre-Reformation English sacred music because in England at that time liturgical practice was dominated by usage of the Sarum Rite and Sarum usage was for the Kyrie to be 'troped'. (Troping a text in this context means inserting extra texts into an original – such as the Kyrie, to make it relevant to a particular Saint or Feast – mfi). Because the Kyrie was troped English composers didn't bother to set it because whatever they composed would be replaced with whatever was considered to be appropriate locally for that particular day in the Church's year.
Taverner based the Kyrie Le Roy on a 'square' – a Kyrie melody used at Lady Mass and has woved some remarkably beautiful and involved polyphony around this cantus firmus which he's assigned to the top part supported by the lower voices' polyphony. The alternatim in in this performance come from the Sarum Kyrie Cunctipotens. Enjoy :-).
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