Which tool would be best for ensuring the accuracy of seam lengths?

A seam gauge is specifically designed for measuring and marking seam allowances accurately. It usually features a sliding marker which allows for precise measurements, making it invaluable for ensuring consistent seam lengths throughout a sewing project. This tool is particularly useful for both straight and curved seams, providing a reliable way to measure small increments and ensure that all seams are uniform, which is crucial for achieving a professional finish.

In contrast, a rotary cutter is primarily intended for cutting fabric with precision, not measuring. Shears are used to cut fabric and are not designed for measurement purposes. A layer of cloth does not serve as a measurement tool; rather, it can complicate the accuracy of a seam length if measurements aren’t taken beforehand. The seam gauge stands out as the optimal choice due to its specific function in the realm of sewing measurements.