Dear all, I have recently read the effects of T-duality of both closed and open strings (Zwiebach 2009, Johnson 2003) and found it very interesting indeed. However, I found it difficult to understand the concept of Wilson lines. Could someone please explain me the idea and concept of Wilson lines for both closed and open strings? How does it work and what is the point of it? Could someone also tell me what is S-duality (g --> 1/g)? How does it actually work? Is there a good review paper on an introduction to Wilson lines and S-duality? If so, where can I find them? Many thanks!