Hi Kuroyume,

Thanks for the help! My understanding is that part of the problem is that the term "gimbal lock" actually refers to two related problems.

1. When parametrizing rotations using Euler angles, the problem is that we are tied down by a specified order of rotations. The advantage of using quaternions is that then there is no rotation sequence, since everything is done in one step.

2. The original gimbal problem has, as far as I understand, to do with the fact that, kind of like in Poser, the gimbals are different. If we look at the picture of the plane in < http://www.fho-emden.de/~hoffmann/gimbal09082002.pdf >,
   the yaw is linked to the table, so it is global, while the roll and the pitch are local.  In this case, the problem is that we get coinciding axes. The roll (local) and the yaw (global) may coincide.

To be honest, I'm not sure if I really understand the picture in
< http://www.hq.nasa.gov/office/pao/History/alsj/lm_imu.gif >,
but I believe that there is a similar thing going on there, namely that the gimbals are somewhat different.

I believe that it is misleading to talk about Euler angles and quaternions when talking about gimbal lock in Poser. In Poser we are not tied to any specific order. However, the y-local, x/z-global design, obviously leads to the same kind of problem as in 2.

So my understanding is that in Poser the problem that can happen is that y and x can coincide or y and z can coincide.

To be honest, I still don't fully understand exactly why the BODY/hip trick works, but it seems that it somehow separates y from x/z.

Thanks!

Helmer