Setting up a forearm roll joint - method 1 of 2

The problem with roll joints is always one of driving them, and that comes down to being able to separate out the twist rotation from the source joint (upper arm/wrist etc. etc.).
The upper arm is pretty tricky in this respect due to the large range of movement and so requires a more complex set up. The forearm, however, is quite a lot easier because the rotation range of the wrist joint within its local coordinate system (of the elbow) is reasonably small.

There are two ways you could set up the twist system. The first is super-quick and 99% stable (but is completely dependent on your wrist joint rotation order for it to work), the second is just as quick but completely stable and doesn't rely on you having optimal wrist rotation orders set.

Actually, if you want to set up a forearm roll joint system I'd definitely do it this way.
The main aim for this post is that it serves as a really great way of exploring rotation orders and the implications they have for your rig. It's quick-to-make practical example of how different orders can affect stability and useablility of the rig and also your own sanity when building more complex systems.

Rotation orders

Rotation orders are perhaps one of the most neglected areas of rigging and yet they have a major impact on the stability of the rig and usability for the animators. Setting the correct orders can save hours of work trying understand curves that make no sense and struggling to clean up wierd rotation pops in a single animation. Multiply that over all animations for a character, all characters in a project and suddenly the importance of getting this right becomes self evident.

The rotation order of a node is basically the order that rotation axis are evaluated. By default Maya creates nodes with an order of xyz which is generally not the optimal setting.