A T-Bone is a special kind of bone that has three joints (normal bone only has two). The extra bone on the T-Bone is called the arm: the arm controls the twist of the bone.

In the figure above, the upward pointing bone is the arm.

Click the Flip button on the Bone panel to change the configuration of the T-bone. Here is the same TBone from above, after flipping once and then again (note that the joints haven't moved, but the bones are flipped):

T-Bones are useful for rigging bipeds, particularly for hips and/or shoulders.