As you know, most Gregorian Psalm Tones have alternative endings. For example, Tone I has 10 possible endings.
The endings are denoted by a letter and a number. I understand (correct me if I am wrong) that the letter itself indicates the degree (on the scale) of the last note of the ending. For example, all the "D" endings end in D while all the "g" endings end in G. The number is just a sequential to differentiate between all the possible endings that end on the same note.
Is there any logic, though, behind capitalizing some of these letters? For example:
- why is a capital D used for the "D" endings of Tone I while a lower case g is used for the "G" endings?
- why is a capital A used for the "A" endings of Tone IV while a lower case a is used for the "A" endings of Tone VII?
I apologize if I am misusing a technical term. I am not a musician or a musicologist.