A special ribbon graph presents a cylinder. I am reading "Quantum Invariants of Knots and 3-Manifolds" by Turaev. I have a dificulty to understand the proof of Lemma 2.6 on page 172.
The lemma says that a special ribbon graph drawn on page 167 presents a cylinder.
I am sorry that I don't know how to show that ribbon graph here.
I especially don't understand the statement starting "One may check that the cylindrical structures are compatible on the boundary..."
Could you show me the detail and/or an intuitive(geometric) proof?
 A: Turaev's book assumes familiarity with basic 3-dimensional geometric topology and especially Dehn surgery presentations of 3-manifolds.  If you want to understand all the details in Tureav's book, then I strongly recommend first reading Rolfsen's "Knots and Links", or some similar text.
It's hard to explain without pictures, but briefly: Start with $S^2\times I$.  Remove a regular neighborhood of the arcs (not the loops) of the tangle in Figure 2.4.  Do Dehn surgery along the (framed) loops.  The boundary of the resulting 3-manifold is the union of a "vertical" annulus for each straight arc of the tangle and "upper" and "lower" surface.  The upper surface contains $S^2\times \{1\}$ (minus some disks) and an annulus for each curved arc of the tangle.  Call this surface $Y$.  Then the 3-manifold, after Dehn surgery, is homeomorphic to $Y\times I$.  Turaev's Figure 2.5 shows a 3-punctured disk which, after Dehn surgery, becomes an instance of curve$\times I$ inside $Y\times I$.
If the above explanation makes no sense to you then you should read Rolfsen.
