What is the best tool to scribe a line parallel to the straight edge of a sheet of metal?

A scratch gage is specifically designed for marking straight lines on materials, including metal. It allows for precise scribing by using a sharp point that inscribes a line when drawn along a straight edge. This tool is particularly advantageous because it can provide a consistent distance from the edge, ensuring that the line is parallel and accurate. The depth of the scribe can also be adjusted based on the thickness of the material, making it a versatile tool for various applications in machining and fabrication.

While a caliper is mainly used for measuring distances or dimensions and not for marking, a ruler, although useful for measuring, lacks the precision necessary to create a marked line directly. It requires additional tools to scribe effectively. A laser level is primarily employed for establishing level or plumb lines over distances and may not be suitable for close-quarters or small-scale applications such as marking metal sheets. Thus, the scratch gage stands out as the most appropriate and effective tool for this specific task.