[Figure 16] The twist of an object, like this joint, should happen as the first axis in the rotation order.

To set the order, instead of thinking in terms of X, Y, Z, think in terms of the motion you are trying to produce. We’ll describe these motions as primary, secondary, and twist. If you think about the joints in your own body, you can determine how these terms relate to the twisting, primary, and secondary directions you will rotate in. Take your arm, for example: at the shoulder, it mainly moves forward and back, but also moves side to side, and twists. At the elbow, the joint can only bend forward and back. Your wrist can bend up/down, twist, and bend a little bit to the sides. With this understanding, you can go through and determine the order in which each part moves. In Maya, you need to determine which axis corresponds to the three distinct motions and set your rotation order accordingly. So how do we know which rotation order is best?

When you stack three rotations in a hierarchy, we basically end up with two easy and predictable rotations and one that tends to screw things up. Let’s build a simple demonstration model.

1. Turn on grid snapping and create a two-joint chain in the front view panel. Place the first joint at the origin, and the second joint to the right of the first joint.
2. Change to the perspective view and select the first joint. Double-click on the Rotation Tool to bring up the Tool Option box and select the Gimbal setting.
3. Keeping the default rotation order of X,Y,Z, using the manipulator, rotate the joint in X. Notice that rotating in the X plane doesn’t affect Y or Z. All three rotation planes in our manipulator are still perpendicular to one another.
4. Now rotate the joint in Z. The X and Y planes rotate with it, keeping the three planes perpendicular.
5. Rotate the joint in Y. The important thing to note is the effect on the X plane as it approaches the Z plane. Our planes are no longer perpendicular — they have begun to orient in the same way. (See Figure 18.)
6. Set the rotation value of the joint back to 0,0,0 and open the Attribute Editor.
7. Under Transform Attributes you’ll see a small menu labeled Rotate Order; change “xyz” to “yzx.”
8. Redo steps 3 through 5. Notice how X rotation now controls Y and Z. Y now affects no other planes, and Z is stuck in the middle, only affecting Y.