But what is STRAIGHTNESS in GD&T?

Types of Straightness in GD&T
Straightness of a Surface (Surface Straightness):

Definition: Ensures that all points along a specific line on a surface remain within a tolerance zone defined by two parallel lines or planes.
Application: Commonly used to control the flatness of a shaft or ensure the smoothness of a guiding rail.
Example Tolerance Zone: Two parallel lines separated by the straightness tolerance (e.g., 0.05 mm).
Straightness of a Line Element (Axis Straightness):

Definition: Ensures that the derived median line (or axis) of a cylindrical feature remains within a cylindrical tolerance zone.
Application: Used for shafts, pins, and holes where the alignment of the axis is critical.
Example Tolerance Zone: A cylinder around the axis of the feature, with a diameter equal to the straightness tolerance.