Thank you, @spikeyxxx, you already brought light into the darkness😀!
For “Quaternions” in Blender, there’s also a good article in the “Blender Art Magazine” with the title “A world of rotations” here. On page 16, the author writes that “W” is the “Rotation Angle” around the axis defined by “X”, “Y” and “Z”.
But your explanation, @spikeyxxx, is obviously the correct one since this is the most precise definition for “X”, “Y”, “Z” and “W” 👍! I think, part of the confusion is that we also have “X”, “Y”, “Z” and “W” in “Axis Angle Rotation”.
Which “Scripted Expression” do I need so that I can use the rotation of a bone around its “Local X Axis” in a driver if the coordinate system is set to “Quaternion (WXYZ)” ?
