Straightedge - An object that provides a linear reference. Used in machine tool metrology for measuring straightness errors. Errors in mechanical embodiments can often be determined and corrected using reversal techniques.

Embodiments:

• Polished edge of a piece of granite or other material.
• Mirrored and polished flat optical surface (usually rectangular in shape).
• Laser light beam.
• Wire that is held taut between two points.