The line where the fabric pieces will be stitched together.
When matching up pattern pieces or making pattern adjustments, it is essential to match or adjust the stitching line, not the cutting edge.
The stitching line is usually 0.5 to 1.5cm from the cut edge. In Commercial Patterns, it is usually a standard 1.5cm (.6 inch) unless otherwise specified. Between the stitching line and the raw edge is the seam allowance.
In this example the stitching line has been marked with a dashed red line. The stitching line is 1.5cm from the raw edge. This gives the pattern 1.5cm seam allowance.
Commercial patterns usually have the stitching line marked like this (except in black, not red).
In the fashion industry the stitching line is usually marked with notches (see Image 2).
This pattern is marked with notches (and drill holes).
In the fashion industry, where there are corners (circled in red on the Bodice Back), it is common practice to mark only one seam-line - notching both corners weakens the pattern.
When making my own patterns, I just notch both corners because generally I won't be using that same pattern over and over again.