Seam Allowance and Shrinkage
Sewn panels don’t join edge-to-edge. They actually join a given distance away from an edge. We call this distance the seam allowance. It’s a personal preference thing but most trimmers sew within about 3/8-1/2 inch from the edge. In other words, they use a 3/8- to 1/2-inch seam allowance. Glasgow uses 3/8 inch for most applications. So the panel shrinks by about 3/8 inch at each edge stitched.
The following illustrates how a panel shrinks due to the seam allowance. The shrinkage due to seam allowance is critical to account for when laying out a job.