bone rig question

[Hoptoad]

I tried every bone-constraint and -parenting variation to get these bones to work like I want, except the elusive correct one. It's for an arm rig needing a special set-up.

The A bone is the upper arm bone, and should rotate like a typical shoulder (360 degrees). The B bone is like a handle used to move the A bone. The B bone should stay at a 90 degree angle to the A bone at all times, and stay connected to the tip of the A bone. The un-identified C bone is the lower arm bone, which should operate as a typical lower arm bone.

Put another way, after I "shy" the A bone, I will only use the B bone to rotate it. The B bone stays perpendicular to the A bone. The lower arm bone rotates as if there is no B bone.

How does a person set this up? I figured-out how to do it a few years ago on a similar rig, but forgot what I did.

Thanks.

[synthsin75]

Easy peasy.
Parent C to B and B to A and angle constrain B to 0,0.

[Hoptoad]

Easy peasy.
Parent C to B and B to A and angle constrain B to 0,0.

Thank you!

I thought the C bone had nothing to do with it, which is why I left it's label off the illustration. Oops!