In "Theory of Elastic Stability" by Timoshenko and Gere, the authors say for a beam in pure torsion,
For a beam of thin-walled open section it can be assumed with reasonable accuracy that the shearing stress at any point is parallel to the corresponding tangent to the middle line of the cross section and is proportional to the distance from that line.
What is meant by the "tangent to the middle line"? A cross section would be a two dimensional shape. For an I-beam, it would be the shape of an I. But I can see two reasonable definitions of what a middle line could be, since an I has two axes of symmetry. And even then, I'm not sure what a tangent to the line is, other than the line itself.