What is a datum line used for in alignment?

• A. A decorative line on the hood.
• B. A reference point used for measuring vehicle alignment.
• C. A line marking the top of the frame.
• D. A measurement unit used for tolerance.

Answer: (B)

A datum line is a fixed reference line used as the baseline for measuring vehicle alignment. In alignment work, technicians establish this line at a known, repeatable location on the vehicle (often along the centerline of the frame or a designated reference point). They then measure from key points on the vehicle to this datum line to determine if the frame, rails, or panels are straight and within spec. If measurements differ from the allowed tolerances, the body or frame may be misaligned and require adjustment. It isn’t decorative, it isn’t a top-of-frame marker, and it isn’t a measurement unit—it's the reference you measure from.