What tool is specifically used for marking hems in fabric?

The seam gauge is specifically designed for marking hems in fabric due to its unique features. It typically has a sliding marker that allows for precise measurement and marking of fabric edges at the desired hem length, which is crucial for achieving consistent and even hems in sewing projects. The seam gauge is particularly useful because it has both a ruler and a movable guide, enabling users to easily set and maintain the measurement while working with fabric.

In contrast, while the scale rule and measuring tape are useful measuring tools, they do not have the feature of a sliding marker, making them less effective for marking specific hem allowances. A sewing machine, on the other hand, is essential for the actual stitching process but does not assist in marking or measuring fabric. Thus, the seam gauge stands out as the most appropriate tool for this particular task.