Let me just break out a thesaurus to see if I can comment on that. Hmm, wait one sec, I might just have to break out my Enigma machine.
Ok, so I think you’re asking if Blender was designed so that the length of the bone was the Y, because Euler XYZ is the most common rotation order.
It’s possible. XYZ is alphabetical and in mathematics it is often taught in that order first.
The middle axis will hit gimbal lock when it reaches +- 90, and Y axis is more often than not the axis that is generally animated the least.
Ton was the one who decided which way all the axes would point many decades ago, and I think it’s possible that was part of his reasoning.
There’s only 1 person who can answer that for sure though.
I know that Euler XYZ is the most natural for an animator because it is the default in every software that I know of. (Blender is odd in the fact that it allows the user to use Quaterions for bones by default)
