What is "straight line accuracy"?

Straight line accuracy is a measurement of the amount of error that a linear positioner will deviate from a perfectly straight line. Straight line accuracy is the error that is in the horizontal plane (x-axis), while flatness is the error in the vertical plane (z-axis). Both are measured at the center of the mounting surface and represent the maximum deviation for the overall length of travel. The straight line value is typically given as the worst case value for both errors.