The seam allowance might appear to be just a border on a shoe upper pattern piece, but it is crucial in determining where two parts will meet after stitching is done. Even a slight variation from one section to another in the seam allowance will cause the assembled upper to look different, even if the actual seam seems correct. Small differences of a few millimeters will impact the form of a vamp, how a quarter meets another part, or how the center line sits. It’s easiest to understand this concept using a simple test piece before going on to assemble an entire upper.
Trace a pair of identical pattern pieces on cheap material. Clearly indicate the intended seam allowance. Carefully cut one pair out. For the second pair, cut the seam allowance wider on one side. Sew the pieces along the seam line. The assembled result will show you a big difference in shape and edge position, which wasn’t obvious on the flat pieces.
When working on an upper assembly, consistent seam allowance is important because several seams interact with one another. The vamp will sew to quarters. The lining will follow roughly the same shape as other parts. Reinforcement pieces may need to sit within a certain area. Each of these pieces must have a uniform allowance. Otherwise, the mismatch will build up as each section is added. What started out as a minor deviation in the cutting process could later become a distorted upper, an uneven opening, or a section that doesn’t lay flat on the shoe last.
Precise marking is helpful, but accurate cutting is just as important. Hold the pattern firmly, use a proper cutting surface, and cut along the marked line without constantly changing the angle of the blade. On curves, slow cutting is often better than trying to make a long cut. After cutting out the mirrored left and right pieces, check them against each other to ensure symmetry. It’s easier to catch an uneven allowance at this stage than after it has been stitched together and is harder to correct.
The placement of the stitches must also match the desired seam allowance. If the seam is supposed to be a certain distance from the edge, stitching it closer or further from the edge changes how much material is left inside the seam. This may alter how the edge is finished, turned, or how thick the seam feels. Don’t judge accuracy by the appearance of the outside face of the seam. Examine the seam from both sides and check to see that the seam allowance is consistent all along.
Once your cutting skills improve, you’ll see fewer issues once the upper gets to the lasting phase. Parts will align more accurately, it will be easier to line up center marks, and you’re less likely to end up with a pair of shoes that have subtly different shapes because of tiny differences added together. Before starting the next phase of the assembly, touch each edge of the cut pieces and compare matched pairs. If the seam allowance already appears uneven before stitching, the seam won’t be any more accurate afterward.
