Thanks. Actually, when I tore apart the old conical Burke and James bellows I had, I saw the trapezoidal stiffeners ran into the corners. The difference was that an entire side panel would have pairs of trapezoids and the other two sides didn't extend into the corners. (The stiffeners on those two sides were also trapezoidal rather than rectangular but with angles more like 75+15+90 for the right triangle.) I could see why one wouldn't want the stiffeners to extend all the way into the corners for all four panels because the thickness of the corner folds would almost double in thickness as a result. However, I couldn't see any reason not to alternate the trapezoidal pairs with the rectangular pairs other than it took a bit longer to lay down the alternating pattern. I thought the alternation might add some additional stability for both the vertical and horizontal panels and also make the corners more stable. It may have added to the ease at which the folding occurred. I don't know this for sure since I've never made another bellows previous to this one where I could compare methods. I expected the folding to be the most difficult part based on what I've read, but I was surprised at what turned out to be the easiest part of the construction.
Originally Posted by argus