This is a topic that has generated much confusion. Most posters give very unintentionally misleading and confusing information. I think that A. Harris gave the best explanation. Feel the fabric at the middle of one of the sleeves (not the lining, just the wool). Then, feel the fabric at the chest, just above the pocket. A fused suit's fabric at the chest area will feel slightly "thicker" -- in a good fused suit only just a little bit -- than the fabric at the sleeve. This is due to the fusible fabric that has been literally "fused" to the on the inside face of the wool fabric. (Imagine a giant iron on patch, only much of much more supple material). Even in a fused suit you will feel a distinct third layer at the chest area. This is the canvas chest piece, and I have never seen any decent suit (read: over $250 retail) that didn't have a "floating" chest piece. Some posters here -- for example, Bjorn H above -- have stated that the difference between a fused and a canvas suit is that in the former the chest piece is "glued" in. I doubt this is the case. I have had one experience deconstructing a fused suit, and the chest piece was most definitely stitched in to the jacket (the stitching was at the perimeter, such that the interior of the canvas was floating). I can't imagine how the chest piece could be glued in. Such a suit would be unimaginably cheap and would likely get ruined in a matter of months. It seems to me that in a canvas suit the "chest piece" is actually a full front piece. That is, the canvas layer runs all the way down to the bottom of the jacket. In a fused suit, because the canvas only runs down to about the pocket level, the manufacturer must use a fusible material to make the lower half of the jacket have shape. Because it would look silly to have only the lower half of the jacket fused (I imagine you could see a line if this were the case), the fusing runs all the way up. This is why even the chest area's fabric will feel thicker. Additionally, this is why another good way to tell if it is fused or not is to feel the bottom half of the jacket -- if there is a distinct third layer near the bottom hem, chances are its canvassed. Also, note that even fused suits will often have canvas in the lapels that is padstitched in. Hart Schaffner and Marx is a good example of this. Though the Gold Trumpeteer is a fused suit (and in my opinion a good one), the lapels contain handstitched canvas. In my opinion, the canvas lapels are very important to look for as well.