One of Ignatius’s more mysterious visions was “seeing” the Holy Trinity in the form of three musical keys.
. . . while praying the office of Our Lady on the steps of [a] monastery, his understanding began to be raised up, in that he was seeing the Most Holy Trinity in the form of three musical keys (en figura de tres teclas), and this with so many tears and so many sobs that he could not control himself. And on walking that morning in a procession which was leaving from there, at no point could he restrain his tears until the mealtime, nor after the meal could he stop talking, only about the Most Holy Trinity.
Michael Peppard draws on his experience as a choral singer to explain what Ignatius meant, and concludes:
The musical triad, then, is a fitting figure for the Trinity. It embodies distinction-in-unity and dynamic unchangeability. What is more, a triad is a trinity that emanates more being. In the second and third centuries, the “light from light” figure — enshrined in the fourth-century creeds — captured the concept of undiminished giving, how a flame could give rise to another flame without itself being diminished. But the musical triad might capture that concept even more aptly: it is the triad’s very self-relationality which generates the overtone sequence. In other words, a musical trinity is a creator, or a begetter.