What does the solid line on pattern pieces indicate?

The solid line on pattern pieces signifies the cut line. This line indicates where the fabric should be cut to create the piece needed for a sewing project. It is crucial for ensuring that the fabric is accurately shaped according to the design. Following the cut line allows for the correct dimensions and shapes to be achieved, facilitating a proper fit when the pieces are sewn together.

The other options represent different functions in sewing patterns: the stitching line typically shows where the sewing should occur, the grain line indicates the direction of the fabric's grain, which is important for the fabric's drape and stretch, and the fit line is used to indicate adjustments for tailoring the garment to achieve a better fit. However, the solid line's primary role is to guide the user on where to actually cut the fabric.