Straightness Error - A lateral translation error (for a point from a line) as a carriage is moved along it's axis.   The error along the nominal line of motion is referred to as linear displacement error. See also straightness,  geometric errors, machine tool metrology, degree-of-freedom.

• Moving Reference Straightness - The straightness of a fixed point associated with the stage base is measured with respect to the reference surface of a straightedge placed on the stage carriage.
• Fixed Reference Straightness - The straightness of a fixed point associated with the stage carriage is measured with respect to the reference surface of a straightedge fixed to the stage's base.

Notation - often the lowercase greek symbol delta is used as a parametric representation of the value in equations. Representations for the straightness in the x direction as a function of nominal motion in the y direction are:
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For these representations the reference coordinate system must be defined or understood.